[docs] Migrate Plugin_contribution

[sarjona]

The URL of the page on https://docs.moodle.org/dev/

https://docs.moodle.org/dev/Plugin_contribution

Is this documentation specific to a Moodle version?

No

What location would you suggest in the new docs?

There are several pages associated with https://docs.moodle.org/dev/Plugin_contribution, like https://docs.moodle.org/dev/Plugin_types or https://docs.moodle.org/dev/Plugin_contribution_checklist).

I would suggest adding most of them to /general/development/plugins/ but link them all inside Guides/Developer guides/Plugins :-)