[llvm][docs] Bump release number from 12 -> 13

[DavidSpickett]

Summary

This seems to have been forgotten. The result was the title
of pages like https://llvm.org/docs/ReleaseNotes.html

Was:
<title>LLVM 13.0.0 Release Notes &#8212; LLVM 12 documentation</title>

[tstellar]

LGTM.

[hans]

There is more than one conf.py file to bump (maybe this could be fixed..).

Last time I bumped the version number I did it like this:
https://github.com/llvm/llvm-project/commit/7ab7b979d29e1e43701cf690f5cf1903740f50e3
https://github.com/llvm/llvm-project/commit/271d9c507c783b3c13629376f3edf66099937938

[DavidSpickett]

I'll cross check with those commits and update as needed.

[DavidSpickett]

Checked all the files in those commits and all conf.py. All set to 13. (apart from the ones that don't set version at all)

llvm/docs/conf.py

 project = u'LLVM'
 copyright = u'2003-%d, LLVM Project' % date.today().year
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
 # built documents.
 #
 # The short version.
-version = '12'
+version = '13'
 # The full version, including alpha/beta/rc tags.
-release = '12'
+release = '13'
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
 #language = None
 
 # There are two options for replacing |today|: either, you set today to some
 # non-false value, then it is used:
 #today = ''