No pages warning

[sofiaromorales]

Bug/issue #, if applicable: #553

Summary

This pr is intended to provide a solution (or at least a better UX) for the cases when a user tries to convert documentation via CLI that results in no pages. 

Documentation with no pages occurs when a user creates article-only documentation and doesn't set a technology root inside the catalog. When this happens, the resulting documentation will show a blank page with an An unknown error occurred. message. This behavior could be frustrating to the users that are just starting with DocC and are not aware of the TechnologyRoot directive.

To improve the user experience, we are emitting a warning both for docc convert and docc preview letting the user know that the resulting documentation doesn't contain any pages and pointing them to the technologyroot doc.

This is the implementation of one of the solutions proposed by @d-ronnqvist in #553, and a topic in the Swift forums will be created to discuss with the rest of the community if this is the most convenient approach or if we should implement a more disruptive solution.

This pr also includes small changes to fix rdar://112462434.

Dependencies

N/A

Testing

Preview and/or a .docc catalog with no technology root. Check the CLI warnings and assert that the following message is printed on screen:

warning: The generated archive does not contain any pages.
For article-only documentation add a Technology Root using the `TechnologyRoot` directive within a `Metadata` directive (https://www.swift.org/documentation/docc/technologyroot).

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 (N/A)

[d-ronnqvist]

Since this error message is user-facing I feel that it'd be good to iterate on it to make it more actionable (depending on context, the developer may not be presented the explanation).

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

This looks good to me. Thank you

[sofiaromorales]

@swift-ci please test

[sofiaromorales]

Thanks @d-ronnqvist