Reorganize new docs 'guides' section

[neverett]

Summary & Motivation

I'm a fair way through auditing old and new docs content, and have started working on figuring out how to plug the remaining gaps in new docs. To that end, I reorganized the /guides section of new docs to make it easier to move old content to the new site.

Big changes I made in this PR:

• Moved docs from the concepts directory into guides directories (automate, build, deploy, monitor, test). The guides docs already contain a lot of conceptual content; the distinction between the two sections seemed fairly arbitrary and not helpful for readers.
• Added a top-level glossary page that can serve as a quick reference to documentation about key concepts, so we don't lose that aspect of having a separate concepts section.
• Created a directory structure that mirrors the URL structure. It's a bit old-fashioned, but this helps readers orient themselves in the docs, and helps contributors reason about where to put stuff.

How I Tested These Changes

Local build.

Changelog

Insert changelog entry or delete this section.

[graphite-app]

This appears to be a placeholder (TK) that needs to be replaced with a link to the migration documentation. Consider updating this before merging to ensure users can find the correct migration guide.

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