[docs] Migrate Database_Schema

[Sara Arjona]

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

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

Is this documentation specific to a Moodle version?

Yes

What location would you suggest in the new docs?

What about /docs/database-schema or docs/guides/database ?

[Sara Arjona]

Implemented in #482