Add monthly active users documentation

[Half-Shot]

Fixes #13599, #10860

This document attempts to describe how MAU is calculated in Synapse, and what the effects are of reaching the MAU limit. It's quite a complex beast in that there are a bunch of configuration options that affect behavior, but I've given it my best shot anyway.

For reviewers, I've taken my cues from the following files:

• https://github.com/matrix-org/synapse/blob/e24ff8ebe3d4119d377355402245947f7de61c00/synapse/app/phone_stats_home.py covers metrics calculation, as well as when the MAU values are updated.
• https://github.com/matrix-org/synapse/blob/4f57ef0b1810bef6071d23e32785623de8b48fdf/synapse/storage/databases/main/monthly_active_users.py covers the core logic of what constitutes MAU.

For determining what would cause a user to become active, I traced the call path of populate_monthly_active_users which ended up at get_user_by_req, so it's fairly safe to assume any authenticated endpoint will be updating MAU.

For determining what actions are blocked by maximum MAU being reached, I looked for any pieces of code calling check_auth_blocking (which is not called at the rest handler level, so it was hard to tie the behaviors to endpoints so much as actions).

[clokep]

Looks good overall! I nit picked a bit because MAU is confusing and I think it would be worth our time to figure this out once and for all and document it. Do feel free to push back if I'm asking something ridiculous.

[babolivier]

2 additional general remarks (since they apply to multiple parts of the diff):

• "appservice" sounds like insider terminology to me, I would replace it with "application service"
• links such as [Homeserver Usage Stats](/usage/administration/monitoriing/reporting_homeserver_usage_statistics) don't work when reading the docs from the Synapse repo (as opposed to the docs website). I think mdbook does the right thing when you link to the markdown file (in terms of translating the path to what it would look like on the website) when building the docs so that sounds like a solution to have such links work on both platforms.

[babolivier]

Almost there!

[babolivier]

I think this is still wrong on the shadow banned front. If a shadow banned user sends a request, we still record them as active, and later down the line we cancel out their action (in a way that makes it look like it succeeded).

[Half-Shot]

I must have misunderstood you initially. I think that makes loads of sense.

[Half-Shot]

I don't think it's worth talking about this at all, they're effectively the same as regular users and I think the absence of any text tells you as much.

[babolivier]

It could be good to document that shadow-banned users count as active users even after the shadow ban has been issued, but not going to block the PR on this.

[clokep]

LGTM, minus a couple of typos.