Add documentation on how to create new connectors

[tharsheblows]

Feature Request

Add documentation on how to create new connectors for plugin authors and developers.

Support forum links:

• https://wordpress.org/support/topic/feature-compatibility-with-polylang-pro/
• https://wordpress.org/support/topic/support-maipoet-plugin/
• https://wordpress.org/support/topic/add-support-for-sensei-plugin/

[delawski]

Related: #819

[delawski]

It looks like we already have a quite comprehensive documentation on how to create custom connectors in the wiki:

https://github.com/xwp/stream/wiki/Creating-a-Custom-Connector

[delawski]

I have updated the https://github.com/xwp/stream/wiki/Creating-a-Custom-Connector page contents with a further polished (thanks to GPT) connector example.