Enable "mentioned in" feature by default

[d-ronnqvist]

Bug/issue #, if applicable:

Summary

This enables the "mentioned in" feature by default and removes the "experimental" word from its feature flag.

Dependencies

None

Testing

• Run docc convert --help

  • The mentioned feature should not be described as experimental

• Build documentation for some project with a symbol that's linked to from the content of some article

  • The symbol's page should display a "Mentioned In" secretion that links back to the article.

• Repeat step 2 but pass --disable-mentioned-in.

  • The symbol's page should no longer contain a "Mentioned In" section.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

This Swift Forums post proposes that we enable this feature by default.