Expand and edit the recipe user guide
Expand and edit the recipe user guide

In this change:
 - Change the overall subheading hierarchy.
   Previously all sections were sub-sections of "intro"
 - Remove the TOC in the intro section, there's already a TOC above
 - Rephrase some headings
 - Add a few sentences to clarify some parts
 - Plus minor capitalization/punctuation/space editing

This doesn't add anything from old_user_guide.md, although I'd like
to do that in a separate CL.

Bug: 1147490
Change-Id: If97e3647f885526ec18bca6de87347a2c9a6b0e3
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/2529234
Commit-Queue: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>
Reviewed-by: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Auto-Submit: Quinten Yearsley <747cf52e0cef2e3e9371802a2d344b6c87126a4b@chromium.org>

luci/recipes-py,luci/recipes-py