Add new logo and diagram to Sphinx/RTD docs

[LauWien]

This PR adds the new message_doc logo to the documentation and is the corresponding PR to https://github.com/iiasa/message_data/pull/368, https://github.com/iiasa/message_ix/pull/597, https://github.com/iiasa/message_doc/pull/53 and https://github.com/iiasa/message-ix-models/pull/68.

How to review

• Read the diff and note that the CI checks all pass.
• Run a specific code snippet or command and check the output.
• View the RTD docs preview at https://docs.messageix.org/projects/ixmp/en/feature-new-logo/ look at:
  • the combined IIASA-message-ix-models logo top left,
  • the favicon
• comment

PR checklist

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

• [x] Update release notes

[codecov[bot]]

Codecov Report

Merging #446 (b7aad6e) into main (2a90e38) will not change coverage. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #446   +/-   ##
=====================================
  Coverage   98.6%   98.6%           
=====================================
  Files         42      42           
  Lines       4441    4441           
=====================================
  Hits        4379    4379           
  Misses        62      62