sysadmin#3 - Rework "Get the code" and "Run the installer"

This is primarily motivated by sysadmin#3 (closed) - i.e. driving toward the new installer UI on D7/BD. But it goes a bit further to also update D8/D9.

General themes:

• Use more collapsible admonitions (with more explanatory headings) to address variations/edge-cases/details
• Try to keep the main body of each major section to a couple mini-paras.
• In similar CMS pages (D7<=>BD; D8<=>D9), provide similar organization of subsections
• Consolidate the discussion about using separate DBs. This is an optional topic (more appropriate to larger deployments - less appropriate to smaller/evaluator deployments), and each page should hyperlink it.

It may be a little difficult to read the diffs -- probably easier to either:

1. Go to temporary site: http://spades.undo.it:8008/backdrop/, http://spades.undo.it:8008/drupal7/, http://spades.undo.it:8008/drupal8/, http://spades.undo.it:8008/drupal9/

2. Checkout the repo and run mkdocs serve, eg

docker run --rm -v "$PWD:/docs" -p 8000:8000 -w /docs "mjcoltd/civicrm-docker-mkdocs" serve --dirtyreload -a 0.0.0.0:8000

Ping @MikeyMJCO @seamuslee @herbdool