Documentation subproject cross-referencing needs to be more sophisticated

[stichbury]

Description

At present, it's not possible to cross reference between subprojects if you need to include a permalink to a subhead.

So it's fine to link to a page:

By convention, a {doc}`pipeline can be defined as having different layers<kedro:resources/glossary>`. 

But not to a header on a page.

By convention, a {doc}`pipeline can be defined as having different layers<kedro:resources/glossary#data-layers>`. 

@astrojuanlu has diagnosed this as resulting from the fact there's no explicit label defined for a section, so Sphinx didn't generate a std:label anchor to it and what you're trying to do won't work.

He says that the way to fix that would be to change the Kedro docs themselves by either:

• manually labeling sections with (heading-target)= https://myst-parser.readthedocs.io/en/latest/syntax/cross-referencing.html#creating-explicit-targets or
• enabling implicit automatic labeling https://www.sphinx-doc.org/en/master/usage/extensions/autosectionlabel.html

For now, I'm retaining the hard-coded link old-school style but this should be fixed in due course.

[astrojuanlu]

This is still a priority, but I'm releasing @stichbury from doing it