Update doxygen file

[chapulina]

🦟 Bug fix

Summary

I noticed warnings like this while building on Jammy:

warning: Tag 'TCL_SUBST' at line 237 of file '/home/chapulina/dev_jammy/ws_garden/build/gz-fuel_tools8/api_tagfile.dox' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"
warning: Tag 'COLS_IN_ALPHA_INDEX' at line 1044 of file '/home/chapulina/dev_jammy/ws_garden/build/gz-fuel_tools8/api_tagfile.dox' has becom
e obsolete.                                                                                                                                          To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"                        
warning: Tag 'TCL_SUBST' at line 237 of file '/home/chapulina/dev_jammy/ws_garden/build/gz-fuel_tools8/api.dox' has become obsolete.
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"                        
warning: Tag 'COLS_IN_ALPHA_INDEX' at line 1044 of file '/home/chapulina/dev_jammy/ws_garden/build/gz-fuel_tools8/api.dox' has become obsole
te.                                                                                                                                         
         To avoid this warning please remove this line from your configuration file or upgrade it using "doxygen -u"

How well do we want support people generating docs locally? Ultimately, what really matters are the docs we upload to https://gazebosim.org through our nightly uploads. As long as we support that use case well, I think the least we should do for other users is:

• Document how to generate docs locally - and if that requires some specific Ubuntu version, say so
• Don't print warnings when they build on any supported platform (for ign-cmake2 that's Bionic, Focal and Jammy).

---

So I ran doxygen -u doc/doxygen/api.in on Focal (which has doxygen version 1.8.17), and the file was automatically updated. I manually modified a few things that seemed to have been explicitly set by us.

My goals are to:

• [ ] Use a newer version of doxygen to generate our docs. I think Focal makes sense, because it's supported by Citadel, Fortress and Garden. This needs to go with https://github.com/gazebosim/docs/pull/274 and I need to test it locally.
• [ ] Remove warnings on Jammy - this isn't addressed yet

Just opening this for visibility for now.

Checklist

• [x] Signed all commits for DCO
• [ ] Added tests
• [ ] Updated documentation (as needed)
• [ ] Updated migration guide (as needed)
• [ ] Consider updating Python bindings (if the library has them)
• [ ] codecheck passed (See contributing)
• [ ] All tests passed (See test coverage)
• [x] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[chapulina]

I actually just realized that our docs don't build on Jammy for gz-sim. Commenting out ADDITIONAL_INPUT_DIRS fixes the build. Something to look into. It's probably related to the doxygen version.

[chapulina]

I actually just realized that our docs don't build on Jammy for gz-sim.

Ok, I remembered now that @j-rivero had run into this, here's the issue:

• https://github.com/gazebosim/gz-sim/issues/1409