Reorder requirements docs into the basics section and move devloping civicrm into core and add backdrop in

This way MkDocs will be able to report errors for broken links

Done with:

perl -0777 -i -pe \
's@(?<!!)\[([^\]]*)\]\((?!https?://)(?!mailto:)(/(?:(?:(?:/(?![\)#]))|[^/)#])(?!.md))+)/?(#[^)]*)?\)@[$1]($2.md$3)@gm' \
`find . -name '*.md'`

I've seen a incidents where an extension developer tries to hardcode paths
(based on their knowledge/understanding of the file-structure).  This is
usually a bad idea, so I'd like to expose the programmatic way as part of
the docs.

When we first ported to Backdrop, Jen+Nate wanted the default file structure
to be a bit different from Drupal (e.g.  `/civicrm.settings.php` instead of
`/sites/default/civicrm.settings.php`; `/modules/civicrm` instead of
`/sites/all/modules/civicrm`).

I think the Backdrop folks have a slightly different approach toward
multisite than the D7 folks: IIRC, the multisite stuff was still there
functionally, but it wasn't the default.

https://wiki.civicrm.org/confluence/display/CRMDOC/Recommendations
(and wiki redirect to go with it)

https://wiki.civicrm.org/confluence/display/CRMDOC/Before+you+start

https://wiki.civicrm.org/confluence/display/CRMDOC/The+developer+community