documentation: split feature flags table to subsections

[TillJan]

What this PR does / why we need it:
for improved navigation to specific feature flags (see rendered preview)

Which issue(s) this PR closes:

• Closes Can't link to individual feature flags in the guides #12005

Special notes for your reviewer:
the feature flag dataverse.feature.only-update-datacite-when-needed is outside the subsections. Should it be moved or mentioned?

Suggestions on how to test this:
none

Does this PR introduce a user interface change? If mockups are available, please link/include them here:
none

Is there a release notes update needed for this change?:
none

Additional documentation:
none

[pdurbin]

I added refs to all the feature flags and edited various pages to link directly to them (rather than linking to the Feature Flags header).

@TillJan thanks! Merging!