Follow-up to #7313 - replace explicit ref links to headers with automod

[rossbar]

#7313 fixed many missing references in NX's objects.inv database by explicitly linking module names to section headings in the docstring.

Another pattern that could be considered is moving the autosummary's for those to the corresponding module docstrings and then using automod in the reference guide to automatically generate everything, and remove any possibility of collision of link names.

To be clear - the current pattern is fine and there shouldn't be any issues! This is just an issue to capture the idea for a minor refactor, it is neither a bug nor high-priority.