Expand `dart doc` with information from dartdoc README and wiki

[timsneath]

Page URL

https://dart.dev/tools/dart-doc

Page source

https://github.com/dart-lang/site-www/tree/main/src/content/tools/dart-doc.md

Describe the problem

https://github.com/dart-lang/dartdoc contains a lot more documentation that is missing from the dart doc page.

Note: This information is almost entirely from the Advanced section.

Expected fix

We should move this across or at least link to it.

Additional context

No response

[parlough]

As a note for when we work on this, the old dartdoc wiki has a lot of content that would be good to formally document as well. Some pieces may not yet be stable/guaranteed enough to include to on dart.dev, but a lot of it would be great.

• Comment references and their syntax
• dartdoc directives

For comment references in particular, refer to dart-lang/sdk#50702 for current/future work in that area.