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