Add monthly active users documentation (#13617)

* Add monthly active users documentation

* changelog

* Tidy up notes

* more tidyup

* Rewrite #1

* link back to mau docs

* fix links

* s/appservice|AS/application service

* further review

* a newline

* Remove bit about shadow banned users.

I think talking about them is confusing, and the current text doesn't imply they get any special treatment.

* Update docs/usage/administration/monthly_active_users.md

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Update docs/usage/administration/monthly_active_users.md

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Co-authored-by: Brendan Abolivier <babolivier@matrix.org>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>