Migrate in Settings Reference Document

Migrate "Extension Reference" wiki page

- `<classloader>`
- `<psr4>`
- `<requires>`
- `<ext>`

I don't know what these do, so I just added them with "not yet
documented"

This is already here:
https://docs.civicrm.org/dev/en/latest/extensions/#extension-names

This is already here:
https://docs.civicrm.org/dev/en/latest/extensions/publish/#prerequisites

https://wiki.civicrm.org/confluence/display/CRMDOC/Extension+Reference

auto-import with no edits

Migrate "Cache reference" wiki page

https://wiki.civicrm.org/confluence/display/CRMDOC/Cache+Reference

Auto-import with no edits

fix typos in interfaces.md

Add in Documentation of APIv3 Interfaces

Add in document on APIv3 Examples

Clarify running automated installation as root

update return outputs (minor editing)

Make the warning about running Ubuntu installation as root slightly more dire.

Add in content on running upgrade-test in CiviBuild to address issue 193

Adding in more detailed reference -> to PHPStorm set up (with screens…