Clean up CI & miscellaneous updates for 3.10

[khaeru]

Parallel to iiasa/ixmp#555, plus:

• Adjust 'tutorial' jobs in 'pytest' CLI workflow to align with Run "pytest" GHA workflow on pull_request_target #909, Validate CI workflows #910.
• Bump copyright year.
• …items below.

How to review

• View the revised docs, in particular:
  • Mention of the newsletter on the index page.
  • Organization of the index page and navigation pane.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
  • Mention newsletter sign-up.
  • Correct outdated links embedded in GAMS code.
• Update release notes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.6%. Comparing base (e0f2e02) to head (fe2086f).
Report is 9 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #911      +/-   ##
========================================
+ Coverage   24.0%   95.6%   +71.6%     
========================================
  Files         48      48              
  Lines       4394    4394              
========================================
+ Hits        1057    4204    +3147     
+ Misses      3337     190    -3147     

see 45 files with indirect coverage changes

[khaeru]

• Mention newsletter sign-up.

@glatterf42 while working on this, I noticed that we have some toctree entries in the documentation that do not appear in the rendered docs:

message_ix/doc/index.rst

Lines 138 to 140 in e0f2e02

	governance
	sharing
	security policy

This is because:

• These files are in the top-level of the repo, outside the doc/ directory, and thus not visible to Sphinx without a relative path.
• These are in Markdown, not ReST, and the file names are in lower case while the references are in upper case.

There is some good and useful text in these files, so I think they should be findable by people clicking around the documentation.

Based on your understanding of GitHub (or more general) practices, can you clarify:

• Is it required that these files be where they are?
• Is it required that the files be in Markdown format?

If yes to both, we can cope, but if no, then we I can very easily make sure they are integrated to the docs.

[glatterf42]

Thanks, I read up in the docs:

• Is it required that these files be where they are?

No, they can be in the root directory, .github/, or docs/. Unfortunately, though, I think this truly means docs/, i.e. our doc/ folder will likely not work.

• Is it required that the files be in Markdown format?

Yes, I think so. In all examples and guides in GitHub's docs it is emphasized that the filename must be in all-caps and the ending .md.

While looking at the docs, I noticed that the community profile seems to not care about the GOVERNANCE.md file anymore. So I think it's entirely up to us what to do with that one.

[khaeru]

Thanks. For the moment I tried to ensure all the contents are linked from the docs index, and also added direct links to the CoC and discussions to the docs sidebar.

[glatterf42]

Thanks, looks good to me :)