Reorder requirements docs into the basics section and move devloping …

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 c…

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

Add in upgrade reference documents

fix typo in rebuild URL

Update phpunit.md to document PHPUNIT_TESTS functionality

Update phpunit.md to clarify requirements for running tests

Improve buildkit installation docs

docs/standards/sql.md - Various improvements for CRM_Util_SQL_Select

Document new changes in CRM_Utils_SQL_Select

- 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