Add `.utils.sphinx_linkcode_github`

[khaeru]

Based on this code example, this extends sphinx.ext.autocode and sphinx.ext.linkcode to link to file source on GitHub.

This will obviate the need to use sphinx.ext.viewcode on ixmp stack repositories.

How to review

• View the built docs for this branch.
• Try some of the [source] links.
• Ensure these work.
• View the docs for the new module, ensure they are complete.
• Note the CI checks all pass.

PR checklist

• [x] Continuous integration checks all ✅

• [x] Add or expand tests; coverage checks both ✅
• [x] Add, expand, or update documentation.

• [x] Update release notes.

[codecov[bot]]

Codecov Report

Merging #459 (3074cd1) into main (13b73e6) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #459   +/-   ##
=====================================
  Coverage   98.6%   98.6%           
=====================================
  Files         42      42           
  Lines       4467    4468    +1     
=====================================
+ Hits        4405    4406    +1     
  Misses        62      62           

Impacted Files	Coverage Δ
ixmp/backend/__init__.py	100.0% <ø> (ø)
ixmp/backend/io.py	98.5% <ø> (ø)
ixmp/utils/__init__.py	99.5% <ø> (ø)
ixmp/testing/__init__.py	95.9% <100.0%> (+<0.1%)	:arrow_up: