Ok, not everyone uses PhpStorm. But there really wasn't much content on
this page at all.

Some discussion here about what to do with this content:
https://github.com/civicrm/civicrm-dev-docs/pull/58

I'm choosing to redirect instead of delete the page because I think that
will benefit the majority of readers (who are likely to be using
PhpStorm).

Migrate "Add-on Formats" wiki page

https://wiki.civicrm.org/confluence/display/CRMDOC/Add-on+Formats

Add in some documentation on classloader and psr4

Add redirect to API Chaning page

Fix redirect

Migrate Region Reference from wiki

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