docs: Move completion tutorial out into its own document

[faho]

This is useful, but it's a rather large thing that's uninteresting to
many users. So we put it into its own (top-level, for now) document.

The reason for this PR is that we have a note somewhere saying that we shouldn't have more separate documents (might have been in the old doxygen docs somewhere), but to be quite honest I disagree with this.

And not just for this one case, tho this is egregious, but also potentially for more things. Not that I'd want one document per "chapter", but the 7 docs we have after this change (and that's counting the license and design docs) are too few.

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• [N/A] Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.md

[ridiculousfish]

I very much agree with this direction, the index file is a wall of text and we should break it up. LGTM!