docs: fix some warnings

[minijackson]

Fixed some of the Sphinx warnings in the documentation.

Some warnings still remain, but I don't know what to do about them:

.../phoebus/docs/source/converter.rst: WARNING: document isn't included in any toctree [toc.not_included]
.../phoebus/docs/source/convertor.rst: WARNING: document isn't included in any toctree [toc.not_included]

The converter.rst and convertor.rst seems awfully similar and neither are included in the documentation, using the toctree directive. I don't know which spelling to use nor where to include these documents in the documentation. If someone can direct me, I can add this fix in this PR.

Checklist

• Testing:

  • The feature has automated tests
  • Tests were run
  • If not, explain how you tested your changes

• Documentation:

  • The feature is documented
  • The documentation is up to date
  • Release notes:
    • Added an entry if the change is breaking or significant
    • Added an entry when adding a new feature

[shroffk]

ping!! @minijackson

[minijackson]

@shroffk yes? Is there something I need to do on my end?

[minijackson]

If you're wondering why I'm removing the formula_functions reference, this file was deleted in 2023 in this PR: #2794

[shroffk]

Ah,
I thought it was a rename... in any case I will accept this PR and add and link a new page for formula functions

[minijackson]

@shroffk I was initially confused by your ping, because I didn't see any other message in this PR. But Loïc told me you had had comments? It's possible those comments are still "pending" and you need to submit them.