[GEOT-7154] Sphinx build failure with extlinks (new warnings in Sphinx build)

[aaime]

@jodygarnett could you have a quick look? Sphinx came up with a new warning that was not there before.

Checklist

• I have read the contribution guidelines.
• I have sent a Contribution Licence Agreement (not required for small changes, e.g., fixing typos in documentation).
• First PR targets the main branch (backports managed later; ignore for branch specific issues).
• Avoid Java 9+ split packages.
• All the build checks are green (see automated QA checks).

For core and extension modules:

• New unit tests have been added covering the changes.
• Documentation has been updated (if change is visible to end users).
• There is an issue in GeoTools Jira (except for changes not visible to end users).
• Commit message(s) must be in the form [GEOT-XYZW] Title of the Jira ticket.
• Bug fixes and small new features are presented as a single commit.
• The commit targets a single objective (if multiple focuses cannot be avoided, each one is in its own commit, and has a separate ticket describing it).

[aaime]

And now a new error:

sphinx:
     [echo] Running sphinx-build -D release=28-SNAPSHOT -q -W --keep-going -j auto -b html -d "/home/runner/work/geotools/geotools/docs/target/developer/doctrees" . "/home/runner/work/geotools/geotools/docs/target/developer/html"
     [exec] 
     [exec] Exception occurred:
     [exec]   File "/opt/hostedtoolcache/Python/3.10.4/x64/lib/python3.10/site-packages/docutils/nodes.py", line 654, in __getitem__
     [exec]     return self.children[key]
     [exec] IndexError: list index out of range
     [exec] The full traceback has been saved in /tmp/sphinx-err-74d5y0gp.log, if you want to report the issue to the developers.
     [exec] Please also report this if it was a user error, so that a better error message can be provided next time.
     [exec] A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

I cannot reproduce locally though, using the latest Sphinx (I could reproduce the extlinks warning).
The same changes in GeoServer appear to be working fine:
geoserver/geoserver#5908

[aaime]

We're not the only ones experiencing this last error.

Going to try to peg Sphinx to an earlier version, using Sphinx 5.0 the day it's released it's probably not a good idea :-D.