[docs] Migrate Admin_settings

[sarjona]

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

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

Is this documentation specific to a Moodle version?

Unsure (Default)

What location would you suggest in the new docs?

What about /docs/apis/other/admin-settings.md ? A new category (Other) could be created inside API guides, to place pages that are not strictly plugin types or subsystems

[sharidas]

Created PR: #327

[sarjona]

Thanks Sujith for migrating this! :-)