[doc] added section on generating the html doc

[kuhnel]

Summary

Added a new section on generating the html documentation from the rst/md sources to our documentation.

Background: I wanted to check what my documentation changes would look like on the website and had a hard time finding how to do that. So I wanted to save other folks the effort.

This is based on the discussion in https://github.com/llvm/llvm-iwg/issues/17

[kuhnel]

fixed missing empty line

[tstellar]

Do you want to mention this script: https://reviews.llvm.org/D95284

[kuhnel]

In D107460#2926359, @tstellar wrote:

Do you want to mention this script: https://reviews.llvm.org/D95284

I thought about it, but the patch has not yet landed and I did not want to point to an ongoing review.

So If you land your patch we can add something like:

To create the documentation for a release you can use the script in 
`llvm/utils/release/build-docs.sh <https://github.com/llvm/llvm-project/commits/main/llvm/utils/release/build-docs.sh]>`_.

[kuhnel]

So, let me land this change and once your shell script is ready, we'll add another sentence to the documentation.

llvm/docs/MarkdownQuickstartTemplate.md

 ##### Hopefully you won't need to be this deep
 
 If you need to do fancier things than what has been shown in this document,
 you can mail the list or check the [Common Mark spec].  Sphinx specific
 integration documentation can be found in the [recommonmark docs].
 
 [Common Mark spec]: http://spec.commonmark.org/0.28/
 [recommonmark docs]: http://recommonmark.readthedocs.io/en/latest/index.html
+
+## Generating the documentation
+
+see [Sphinx Quickstart Template](SphinxQuickstartTemplate.html#generating-the-documentation)

llvm/docs/SphinxQuickstartTemplate.rst

......+:.                    ..
             .+:::+::              :
             ..   . .+            ::
                      +.:      .::+.
                       ...+. .: .
                          .++:..
                           ...
 
 
+Generating the documentation
+============================
+
+You can generate the HTML documentation from the sources locally if you want to
+see what they would look like. In addition to the normal 
+`build tools <docs/GettingStarted.html>`_
+you need to install `Sphinx`_ and the 
+`recommonmark <https://recommonmark.readthedocs.io/en/latest/>`_ extension.
+
+On Debian you can install these with:
+
+.. code-block:: console
+
+   sudo apt install -y sphinx-doc python-recommonmark-doc
+
+Then run cmake to build the documentation inside the ``llvm-project`` checkout:
+
+.. code-block:: console
+
+   mkdir build
+   cd build
+   cmake -DLLVM_ENABLE_SPHINX=On ../llvm
+   cmake --build . --target docs-llvm-html
+
+In case you already have the Cmake build set up and want to reuse that, 
+just set the CMake variable ``LLVM_ENABLE_SPHINX=On``.
+
+After that you find the generated documentation in ``build/docs/html``
+folder.