Enhance documentation with info about html_context

[langner]

Recently wanted to investigate the details about html_context for cclib/cclib.github.io#8 and could not find anything. It would be nice to have something in the docs.

[artob]

👍

[Blendify]

This is a duplicate of #231

[jessetan]

This does not seem to be fully covered by #231. Only :github_url:, :bitbucket_url:, and :gitlab_url: are documented, no other options are, nor where to put them. Reopening.

[htgoebel]

Hmm, this was closed as duplicate, then reopend since it is not. Still the label "duplicate" is still present. Shouldn't this be removed?

[peterjc]

Note html_context can be used with the GitHub, Bitbucket etc links - see example here:

#465 (comment)

Update: This wasn't obvious from the issue itself, although I see cclib/cclib.github.io#8 covered it with a very similar example.

[stsewd]

Closing in favor of #582