Add new dark link option and improve inverted link accessibility documentation

As discussed in comments under https://github.com/canonical/vanilla-framework/issues/3350#issuecomment-1072448352 we need to improve how we handle links on dark backgrounds:

- introduce new class name for links on dark background (with new lighter blue colour)
- update current "inverted" link to show an underline (possibly rename it)
- update documentation to describe when to use new dark link and when to use "inverted" link

Start by reading the discussion in issue 3350, especially the linked comment, to understand the intended dark-background link behavior. Review the existing inverted-link implementation and documentation, then define completion as a new dark-link option, an underlined or renamed inverted-link treatment, and documentation distinguishing when to use each.

Written by the indexing model from the issue text.