Refine cref handling

[joshua-light]

Previously, we handled cref only partially.

In this PR, a few improvements and fixes are introduced:

• Now almost all <see cref="..."> are handled properly
• <inheritdoc> also works properly
• References to fields, methods, etc. are properly handled via HTML #... syntax
• References that we can't resolve are now rendered as dummy links (simple underscored text).

Improvements to do:

• Handle standard types.

Closes #44. Closes #30.