docs(medcat): Move away from v2 prefix

[mart-r]

This PR moves away from medcat-v2 prefix to just medcat.

While the change itself is not super massive, it will touch a lot of files that had links or other references to the v2 path.
The most immediate changes are:

• medcat-v2 folder renamed to medcat
• medcat-v2-tutorials folder renamed to medcat-v2-tutorials

But there's a lot of things in many different parts (especially with local install / test against core lib state) that also needed changed.

PS:
This PR will trigger almost all the workflows (didn't check, but just a guess - it targets most of the larger ones at least). So I wouldn't be surprised if something is broken somewhere. Especially if it hits stuff that we've not touched in a while.

EDIT:
The docs build initially failed since the config on their side says to explicitly look for medcat-v2/readthedocs.yaml and that can no longer be found. But I did change it (tempoarily) to the new medcat/readthedocs.yaml. If/when this PR gets merged, the change would need to be made permanent.
In any case, here's the link to the docs:
https://cogstack-platform-toolkit--607.org.readthedocs.build/projects/nlp/en/607/

EDIT2:
For some reason a bunch of the links in the docs are not working.
Not entirely sure why that is.
As far as I can tell, I've done everything in the same way that it was done in the previous medcat[-v2]/mkdocs.yaml.
But I'm sure I just missed something obvious.

EDIT3:
I've since resolved the above issue. Turned out the conversion I had done was all good. But the issue was in how some plugin was handling the folder name of medcat/medcat (previously medcat-v2/medcat). Because of both folders having the same name, some plugins didn't work as expected.
So the fix was to add a hook to fix the links.
The reason these needed to be fixed in the first place is because they're in the README.md which is pulled into docs/index.md. However, the links are relative to the medcat folder rather than the medcat/docs folder. Previously this was correctly fixed automatically, but now it was not due to the confusion with the folder names.
The other caveat is that the hook had to change README.md during index.md read since the inclusion of it within index.md happens by another plugin at a later stage.