Also updates all crosslinks including from UI code

Changelog: changed

Rename files in doc/tutorials/ and fix all links.

H1 headers needs to be in YAML frontmatter now

These top-level pages are mostly just a bullet list of links to sub
pages.

Make these diagrams prettier and more accessible. Configure the font
each one uses, and then set up accessible titles and descriptions
for each one.

The UI now says "Code" so let's make the docs match reality.

Update the links in all docs except user and admin pages

Because we want consistency in tutorial
page titles

Because it was still showing top-to-bottom
instead of left-to-right on the live site

Because it was rendering top-to-bottom instead of
left-to-right. Not sure why.

Creates a faux group for Tutorials, so pages with tutorial content
can be routed to Kati for review. Adds the relevant metadata to the
pages in the doc/tutorials directory.

The UX group moved under Product, so these links
need to be updated.

Because the menu changed.
Related to: https://gitlab.com/gitlab-org/gitlab/-/issues/366082/

Links to index files without the index.md in the link
are broken in /help, as well as broken when working
with the docs locally.

These graphs rendered with tiny text when oriented left-right.
Shifting to top-down gives the text more room, and makes it more
legible.

To better incorporate the new Git
tutorial, this MR is meant to help
organize the existing Git content.
It's a small MVC that helps move
in a different direction and remove
duplicate content

And retook earlier blurry screenshot