Also removes some duplicate table of contents entries in maintenance-synapse.md

Related to https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/963

This also simplifies Prerequisites, which is great.

It'd be nice if we were doing these checks in some optional manner
and reporting them as helpful messages (using
`matrix_playbook_runtime_results`), but that's more complicated.
I'd rather drop these checks completely.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/683

ot -> to

This may be especially helpful to existing playbook users
who happen to find themselves installing Go-NEB later on.

instead of bullet points which make it more difficult to tell whether or not you completed all of the steps

This document didn't describe that it is necessary to have a DNS-entry for stats sub-domain.

don't spawn an extra container
run pg_dumpall within matrix-postgres instead, ensures correct version
store under /matrix so a backup of the folder will contain a DB dump
use absolute paths just in case something in the ENV is messed up

All other examples show the config to lower the memory usage on the server

Change markdown to use link instead of tags for Telegram App setup

Adding more presets in the future would be nice.

fixes HTTP 400 Error: "{\"errcode\":\"M_MISSING_PARAM\",\"error\":\"Missing key 'avatar_url'\"}"

Until [this issue](https://github.com/matrix-org/matrix-appservice-slack/issues/348) is fixed

.. and other documentation improvements.

We mainly switch the anonymous metrics viewing variable
to false, along with other wording changes.

Added note about cloudflare, related to #821

Warn users of the known bug https://github.com/turt2live/matrix-dimension/issues/395

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/682