Use indexed option directive on unit reference documentation (Infra)

[tang-mm]

Description

The unit references currently use definition lists for explaining each field, but each element is not indexed so cannot be cross-referenced. This PR replaces those <dl> list and manually defined anchors with the :option: directive in Sphinx

• Field names are defined with .. option:: name, and then are indexed during doc build
  • the defined fields can be cross-referenced by using :option: `name`
• Scope can be defined with .. program:: unit, so that duplicated option names can be specified by adding the scope name before the field :option: `unit name`
• Update CSS style for the generated .sig elements
• Update spelling check settings to ignore field names with .sig CSS class

Chore:

• Enabled fail_on_warning on readthedocs build to prevent broken links

Resolved issues

CHECKBOX-1747

Documentation

NA

Tests

PR checks on Readthedocs
Preview: https://canonical-checkbox--1739.com.readthedocs.build/en/1739/reference/units/job.html

[tang-mm]

Fixed ↓
CSS style behaves differently than my local build, fix needed :(

[Hook25]

lgtm, should we enable fail on warning on all doc sets?

[tang-mm]

fail_on_warning is enabled by default in later versions of the Sphinx template. Checkbox is an early adopter so it was not here, but most other doc sets already have them :)

[Hook25]

do double check on the others if you have some time, I'm pretty sure testflinger doesn't have it

[tang-mm]

Nice reminder! The option was removed from Testflinger two weeks ago 🤔

[Hook25]

now I remember, I approved that PR. The problem with that option is that the build fails on readthedocs but not locally and I couldn't find a way to make it behave the same, which is pretty crappy. Can you please look into that as well? The behaviour should be the same