https://wiki.civicrm.org/confluence/display/CRMDOC/Example+of+creating+a+payment+processor+extension

https://wiki.civicrm.org/confluence/display/CRMDOC/Testing+Processor+Plugins

https://wiki.civicrm.org/confluence/display/CRMDOC/Create+a+Payment-Processor+Extension

https://wiki.civicrm.org/confluence/display/CRMDOC/What+is+a+Payment+Processor

https://wiki.civicrm.org/confluence/display/CRMDOC/Page+Templates

https://wiki.civicrm.org/confluence/display/CRMDOC/Customize+Built-in+Profile+Contribution+and+Event+Registration+Screens

mark `<requires>` and `<ext>` as experimental, and link to the PR which
makes them functional.

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

- `<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

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

Two recent SE questions have made me think that readers are missing this
key set of installing CiviCRM with civibuild (instead of with the normal
installation steps). I've added clarification here which I hope will
prevent future users from stumbling on this step.

ref:

https://civicrm.stackexchange.com/questions/19294/how-to-create-entity-in-civicrm
https://civicrm.stackexchange.com/questions/19194/unable-to-locate-folder-xml-under-the-civicrm-root-directory