APIv3 REST: Document CSRF, backward-compatibility, migration

This updates the docs that correspond to https://github.com/civicrm/civicrm-core/pull/19727. I've posted a preview build. Key pages are:

• New section discussing migration of extern/rest.php to civicrm/ajax/rest
  • https://think.hm/docs/apiv3rest/api/v3/changes/#restendpoint
• New section discussing X-Requested-With and CSRF:
  • https://think.hm/docs/apiv3rest/api/v3/rest/#x-requested-with
  • https://think.hm/docs/apiv3rest/security/csrf/
• All new or modified pages:
  • https://think.hm/docs/apiv3rest/api/v3/rest/
  • https://think.hm/docs/apiv3rest/framework/authx/
  • https://think.hm/docs/apiv3rest/security/csrf/