Migrate and update docs on "API Keys"
The docs on how to create API keys are currently embedded in the developer guide - within the page about CRUD APIs, there is a multipart subsection describing how to administer API keys. (https://docs.civicrm.org/dev/en/latest/api/interfaces/#keys)
This migrates the content over the sysadmin guide - and expands on it.
This lays some groundwork for further cleanup of how authentication is presented.