ref:
https://github.com/civicrm/civicrm-dev-docs/pull/301#issuecomment-324795431

Originally there were different types of extensions - payment processors, reports & modules. For a long time only modules have been recommended and I feel documenting the others only adds confusion.

Migrate payment processor docs from wiki

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

Fix typo in Mkdocs.yml

Import wiki content about Smarty templating

https://wiki.civicrm.org/confluence/display/CRMDOC/Extending+Smarty

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

Rewrite Permissions document a little to cover off the Extensions and…

Migrate "Backbone Reference" wiki page

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

Migrate "Transaction Reference" wiki page

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

Import "manual testing" pages from wiki

https://wiki.civicrm.org/confluence/display/CRM/Pull-Request+Process

https://github.com/civicrm/civicrm-core/blob/master/.github/CONTRIBUTING.md