[ENH] refactoring html representation documentation link to allow custom linking in private and public extensions to sktime

in https://github.com/sktime/sktime/pull/6876, @mateuszkasprowicz has added functionality (similar to sklearn) in the html repr of estimator objects to link to the documentation.

This covers sktime native estimators, but the link will point to a non-existent location for 2nd/3rd party packages (public packages using the sktime interface), as well as private packages.

I think we should give power users and extenders an option to specify the location the documentation link points to, I would approach the design in two levels here:

• a tag, e.g., doc_loc, that is a str with an URL. This would be placed in the "simple" extension templates.
• the option to override _get_doc_link, as an intentional part of the extension template. This should only be used if there are many estimators in the same package, e.g., following a documentation pattern.
  • The advanced (and not the simple) extension templates, and/or the documentation of the doc_loc tag could explain this override pattern.
  • There should be a default _get_doc_link, and it would just get the location from the doc_loc tag.

Finally, once the get_doc_link override point is established, we want to do the following refactor:

• move the mixin up to scikit-base
• change the sktime mixin to be just an override of the get_doc_link method or similar.

FYI @mateuszkasprowicz as the author; FYI @felipeangelimvieira, @tveten as the two most likely individuals known to me who may like to link to separate documentation online.