Improving cross-ref functionality

[choldgraf]

Originally written by @chrisjsewell:

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Basically some code needs to written to (a) check the link is local (e.g. doesn't start with https, etc),
(b) get hold of the file extensions that sphinx deems 'linkable', then (c) if both these check pass, remove the extension.

Thinking about it now though, maybe if these checks parse, then we should also actually check if the file exists and, if it does, then set it as equivalent to writing `doc`{name.txt} here: https://github.com/ExecutableBookProject/MyST-Parser/blob/cf3352c8a224af219cb062cfe467d7da9289284a/myst_parser/docutils_renderer.py#L583

See sphinx.roles.AnyXRefRole for how the ref roles actually work.