Adapt to new RTD configuration style

[glatterf42]

This PR aims to get our current RTD builds in line with the upcoming changes to the required configuration file and prevent future build issues due to dependency changes at the same time. See iiasa/ixmp#491 for a detailed description.

Specific to this PR is the renaming of the config file for internal consistency.

How to review

• Read the diff and note that the CI checks all pass.
• Activate build on RTD for this branch and check that the builds are successful (note: formats other than HTML are too resource-expansive and will only be built after a merge)
• Build the documentation and look at the bullet lists.

PR checklist

• Continuous integration checks all ✅
• [ ] Add or expand tests; coverage checks both ✅ Internal, docs-related, only need them to build/render correctly
• [ ] Add, expand, or update documentation. Only related to config of docs.
• [ ] Update release notes. Only related to config of docs.

[codecov]

Codecov Report

Merging #114 (16d11a4) into main (ec7e57a) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #114   +/-   ##
=====================================
  Coverage   68.4%   68.4%           
=====================================
  Files         63      63           
  Lines       4176    4176           
=====================================
  Hits        2857    2857           
  Misses      1319    1319