Fix Sphinx nitpicks fron message_ix docs

[khaeru]

This PR adjusts some internal intersphinx references.

Where the following would ordinarily be sufficient for an internal reference:

:class:`.Platform`

…this leads to a missing reference when the docstring that contains it is included in the downstream (message_ix) docs.

This PR adjusts some of these references to, for instance:

:class:`.ixmp.Platform`

This has no effect on the appearance of the ixmp docs, but allows the reference target to be correctly identified downstream.

How to review

• Read the diff and note that the CI checks all pass.

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅
• Add, expand, or update documentation.
• Update release notes. N/A, docs details only

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (be06f38) 98.6% compared to head (9579188) 98.9%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #506     +/-   ##
=======================================
+ Coverage   98.6%   98.9%   +0.2%     
=======================================
  Files         44      44             
  Lines       4794    4794             
=======================================
+ Hits        4729    4743     +14     
+ Misses        65      51     -14     

Files	Coverage Δ
ixmp/backend/__init__.py	100.0% <ø> (ø)
ixmp/core/timeseries.py	98.7% <ø> (ø)
ixmp/report/operator.py	99.0% <ø> (ø)

... and 5 files with indirect coverage changes

[glatterf42]

LGTM :)

[glatterf42]

Rebased after dependabot update to main: I suppose this test failure is flaky. Should we include a corresponding marker?