Add CLI flag to enable DocC adding minimal per-page HTML content to each index.html file

[d-ronnqvist]

Bug/issue #, if applicable: rdar://163326857

Summary

This is the last piece of #1366.

It adds a new --experimental-transform-for-static-hosting-with-content flag that enables DocC's feature to supplement the output with minimal content I the per-page "index.html" files.

It also improves the robustness of the modifications of the "index.html" files (in 8e996c4) — even though this is more a theoretical issue since the DocC Render templates have had the same structure for as long as the project has existed.

It also adds support for mixing this experimental feature with the --experimental-enable-custom-templates flag (in 4910bf9).

Lastly, it adds an entry to the "features.json" file so that tools can check if DocC supports this feature before passing the flag.

Dependencies

None.

Testing

from #1366:

• Build documentation for some project and pass the --experimental-transform-for-static-hosting-with-content flag
• Start a basic web server from within the documentation archive, for example ruby -run -ehttpd . -p8000.
  Note: the later steps won't work with docc prevew
• Browse the local documentation.
  • Everything should look and work the same as today
• Disable JavaScript in your web browser
• Refresh the currently open documentation page
  • You should see a very plain version the content, without most of the styling and layout
• Click though links in the content, curation, and breadcrumbs
  • You should be taken to the page that you clicked on

• Navigate to one of the index.html files in the file system and open (it by double clicking).
  Alternatively, on macOS you can run open /path/to/SomeModule.doccarchive/documentation/somemodule/index.html
  • You should see a completely plain version the content, without any of the styling and layout
• Click though links in the content, curation, and breadcrumbs
  • You should be taken to the page that you clicked on

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]

@swift-ci please test

[anferbui]

Mostly got some questions, but overall looking good.

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test

[anferbui]

LGTM, up to you whether you address the nit or not!

[anferbui]

Nit: This test also has the same typo, but up to you if you'd like to fix it!

Suggested change

	This is an _formatted_ article that becomes the root page (because there is only one page).
	This is a _formatted_ article that becomes the root page (because there is only one page).

[d-ronnqvist]

LGTM, up to you whether you address the nit or not!

I can fix it. I just need to update the test expectation to match.

[d-ronnqvist]

@swift-ci please test