[docs] Migrate DB layer 2.0 delegated transactions

[sarjona]

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

https://docs.moodle.org/dev/DB_layer_2.0_delegated_transactions

Is this documentation specific to a Moodle version?

Yes

What location would you suggest in the new docs?

docs/apis/core/dml/delegated-transactions.md

I would also rename the page from "DB layer 2.0 delegated transactions" to "Delegated database transactions" or similar (removing the 2.0 reference). The page will need some love too, because it was marked as a "Work in progress" ;-)

[abgreeve]

I've made a pull request for this issue here - #312