[docs] Migrate /dev/Cache_API to /docs/apis/subsystems/muc/index.md

[tabishnaqvi1311]

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

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

Is this documentation specific to a Moodle version?

None

What location would you suggest in the new docs?

/docs/apis/subsystems/muc/index.md