Create interacting-with-core

Per discussion in documentation channel I think we need to document what methods are considered supported / expected to be stable. The last 4 items in this list have not been previously documented as a supported way to interact with CiviCRM so may be subject to discussion

docs/extensions/interacting-with-core

+There are a number of ways in which extensions can interact with core. These are supported to greater or lesser degrees and only extensions that use fully supported methods are eligible for publication in the extensions directory.
+
+# Fully supported methods are:
+ - civicrm v3 api
+ - civicrm v4 api (when it is stable)
+ - hooks documented in this guide
+ - [Civi::cache](https://docs.civicrm.org/dev/en/latest/framework/cache/)
+ - Civi::$statics (page to be added)
+ - [Civi::settings](https://docs.civicrm.org/dev/en/latest/framework/setting/)
+ - internal symphony listeners  - 'civi.api.resolve', 'civi.api.prepare'
+ 
+