[docs] Document and publish LLVM community calendar

[kristof.beyls]

Summary

Let's introduce and publish an LLVM community calendar.

The idea is that organizers of events such as online sync-ups or office hours
invite [e-mail] to the event they're creating. That way, the calendar
publicly visible at
https://calendar.google.com/calendar/u/0/embed?src=[e-mail] will show
the event.

The hope is that having a single calendar showing all LLVM events makes it
easier for both new comers and experienced people to discover events they're
interested in.

This patch partially implements https://github.com/llvm/llvm-project/issues/55426

We could also give pointers to the calendar in a few other places, e.g. from
the main LLVM page, but let's introduce that incrementally.

[kristof.beyls]

is a screenshot of what the "LLVM community calendar" section looks like after rendering.

[beanz]

This is awesome and a huge win!

Can you also make a PR to the llvm-www repo to get this posted on the main llvm.org page?

[tonic]

This can be another patch, but I would like to add a point of contact to all sync-ups for CoC purposes. This should also be included in the calendar notes. I'm happy to create the patch though.

[kristof.beyls]

In D127852#3593151, @beanz wrote:

This is awesome and a huge win!

Can you also make a PR to the llvm-www repo to get this posted on the main llvm.org page?

Yes, I intend to also create a PR to have this posted on the main llvm.org page.
I'm not sure if on the main page this should simply be a link in the left hand column under "site map" pointing to https://calendar.google.com/calendar/u/0/embed?src=[e-mail], or whether we should also have an embedded calendar on the front page. For now, I think my preference would be to just have the link in the left hand column, but I could be persuaded that having the calendar widget embedded could be good too.

[kristof.beyls]

In D127852#3593826, @tonic wrote:

This can be another patch, but I would like to add a point of contact to all sync-ups for CoC purposes. This should also be included in the calendar notes. I'm happy to create the patch though.

I fully agree it is important that it's clear that the CoC is active for these events and it being equally important that it's clear how to act/enforce it if required.
It seems you have a clearer idea than I have on how to implement that in detail, so I hope it's OK I leave it to you to create such a patch?

llvm/docs/GettingInvolved.rst

 Online Sync-Ups
 ---------------
 
 A number of regular calls are organized on specific topics. It should be
 expected that the range of topics will change over time. At the time of
 writing, the following sync-ups are organized.
 The :doc:`CodeOfConduct` applies to all online sync-ups.
 
+If you'd like to organize a new sync-up, please add the info in the table
+below. Please also create a calendar event for it and invite [e-mail]
+to the event, so that it'll show up on the :ref:`llvm-community-calendar`. 
+
 .. list-table:: LLVM regular sync-up calls
    :widths: 25 25 25 25
    :header-rows: 1
 
    * - Topic
      - Frequency
      - Calendar link
      - Minutes/docs link
* - LLVM Embedded Toolchains
      - `ics <https://drive.google.com/file/d/1uNa-PFYkhAfT83kR2Nc4Fi706TAQFBEL/view?usp=sharing>`__
        `gcal <https://calendar.google.com/calendar/u/0?cid=ZDQyc3ZlajJmbjIzNG1jaTUybjFsdjA2dWNAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ>`__
      - `Minutes/docs <https://docs.google.com/document/d/1GahxppHJ7o1O_fn1Mbidu1DHEg7V2aOr92LXCtNV1_o/edit?usp=sharing>`__
    * - Clang C and C++ Language Working Group
      - 1st and 3rd Wednesday of the month
      - `gcal <https://calendar.google.com/calendar/u/0?cid=cW1lZGg0ZXNpMnIyZDN2aTVydGVrdWF1YzRAZ3JvdXAuY2FsZW5kYXIuZ29vZ2xlLmNvbQ>`__
      - `Minutes/docs <https://docs.google.com/document/d/1x5-RbOC6-jnI_NcJ9Dp4pSmGhhNe7lUevuWUIB46TeM/edit?usp=sharing>`__
 
+.. _office-hours:
 
 Office hours
 ------------
 
 A number of experienced LLVM contributors make themselves available for a chat
 on a regular schedule, to anyone who is looking for some guidance. Please find
 the list of who is available when, through which medium, and what their area of
 expertise is. Don't be too shy to dial in!
* - Anastasia Stulova
     - `GoogleMeet <https://meet.google.com/kdy-fdbv-nuk>`__
     - English, Russian, German (not fluently)
 
 
 Guidance for office hours hosts
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 * If you're interested in becoming an office hours host, please add your
-  information to the list above.
+  information to the list above. Please create a calendar event for it and
+  invite [e-mail] to the event so that it'll show up on the 
+  :ref:`llvm-community-calendar`.
 * When starting an office hours session, consider typing something like "*Hi,
   I'm available for chats in the next half hour at* video chat URL. *I'm
   looking forward to having conversations on the video chat or here.*" on the
   LLVM chat channels that you are already on. These could include:
 
     * the `#office-hours Discord channel
       <https://discord.com/channels/636084430946959380/976196303681896538>`__.
     * :ref:`IRC`

 :doc:`Proposals/VariableNames`
    Proposal to change the variable names coding standard.
 
 :doc:`Proposals/VectorizationPlan`
    Proposal to model the process and upgrade the infrastructure of LLVM's Loop Vectorizer.
 
 :doc:`Proposals/VectorPredication`
    Proposal for predicated vector instructions in LLVM.
+
+.. _llvm-community-calendar:
+
+LLVM community calendar
+-----------------------
+
+We aim to maintain a public calendar view of all events happening in the LLVM
+community such as :ref:`online-sync-ups` and :ref:`office-hours`. The calendar
+can be found at
+https://calendar.google.com/calendar/u/0/embed?src=[e-mail] and can
+also be seen inline below:
+
+.. raw:: html
+
+    <iframe src="https://calendar.google.com/calendar/embed?height=600&wkst=1&bgcolor=%23ffffff&ctz=UTC&showCalendars=0&showDate=1&showNav=1&src=Y2FsZW5kYXJAbGx2bS5vcmc&color=%23039BE5" style="border:solid 1px #777" width="800" height="600" frameborder="0" scrolling="no"></iframe>