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 the PR is the attempt to incorporate the latest version of ixmp/main as a requirement. It has been required on RTD in the past, so I migrated that requirement to the automatically created doc/requirements.txt. This might make ci/rtd-requirements.txt useless, but I kept it for now since I don't know which other workflows might rely on it. Also, this PR renames 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 #733 (75158d1) into main (0b1983f) will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #733   +/-   ##
=====================================
  Coverage   94.5%   94.5%           
=====================================
  Files         43      43           
  Lines       3463    3463           
=====================================
  Hits        3273    3273           
  Misses       190     190