tests: python interlinks

[machow]

This PR is a step towards #110 , by...

• pulling all interlinks logic from the python filter into quartodoc.interlinks
• adding a test specification (quartodoc/tests/example_interlinks/spec.yml)
  • this spec maps inputs like [](`quartodoc.get_object`) to responses (e.g. a link with text X), or elements (e.g. a Code element).
  • this specification is tested on the python side, but remains to be implemented in a lua filter.

Note that the interlinks logic could also be used in the renderer in the future, to avoid the need for a filter at all. However, the filter allows interlinks to work from any page, not just those rendered by quartodoc.