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

Reorder extensions files and rename to be extension structure to be clear its more than just files its directory strcutre and improve doc a bit

- General formatting improvements.
- Change  "Ubuntu 12.04 or 14.04" to "Ubuntu 12.04 or later" because the
  buildkit install script is clearly designed to work for later versions
  of Ubuntu.
- Relocate troubleshoot steps to after configuration steps (so the
  reader doesn't stop reading when they see "troubleshooting".
- Higlight post-installation steps.
- New instructions for manually ading webserver settings after running
  `amp config`.
- More troubleshooting help.

This was in the general "buildkit" page, and I think it fits better in
the specific "civibuild" page.

This is a direct copy-paste from one section of the guide to another,
without any edits. When someone is installing buildkit, these steps are
easy to miss if they're located in the civibuild file.

See this page for correct syntax:
https://docs.civicrm.org/dev/en/latest/documentation/markdown/#admonitions

- Use the GitLab repo instead of the GitHub one
- Provide more instructions for the step where you create a fork

Fix example

The doc mentions the page "Administer => System Settings => Cleanup Caches and Update Paths" as a way to clear caches. This is half-true: it does clear `templates_c` and the `civicrm_cache` table, but there are other things that it doesn't touch. (Compare `CRM_Admin_form_Setting_UpdateConfigBackend::postProcess` with the `CRM_Core_Invoke::rebuildMenuAndCaches`.)

IMHO, `UpdateConfigBackend::postProcess` should be patched to just do a general system flush (`civicrm_api3('System', 'flush', array())`) -- but (for now) the page is misleading and we shouldn't direct people there.