Make systemd-with-workers doc official (#7234)

Simplify and update this documentation, and make it part of the core dist.

docs/systemd-with-workers/README.md

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+# Setting up Synapse with Workers and Systemd
+
+This is a setup for managing synapse with systemd, including support for
+managing workers. It provides a `matrix-synapse` service for the master, as
+well as a `matrix-synapse-worker@` service template for any workers you
+require. Additionally, to group the required services, it sets up a
+`matrix-synapse.target`.
+
+See the folder [system](system) for the systemd unit files.
+
+The folder [workers](workers) contains an example configuration for the
+`federation_reader` worker.
+
+## Synapse configuration files
+
+See [workers.md](../workers.md) for information on how to set up the
+configuration files and reverse-proxy correctly. You can find an example worker
+config in the [workers](workers) folder.
+
+Systemd manages daemonization itself, so ensure that none of the configuration
+files set either `daemonize` or `worker_daemonize`.
+
+The config files of all workers are expected to be located in
+`/etc/matrix-synapse/workers`. If you want to use a different location, edit
+the provided `*.service` files accordingly.
+
+There is no need for a separate configuration file for the master process.
+
+## Set up
+
+1. Adjust synapse configuration files as above.
+1. Copy the `*.service` and `*.target` files in [system](system) to
+`/etc/systemd/system`.
+1. Run `systemctl deamon-reload` to tell systemd to load the new unit files.
+1. Run `systemctl enable matrix-synapse.service`. This will configure the
+synapse master process to be started as part of the `matrix-synapse.target`
+target.
+1. For each worker process to be enabled, run `systemctl enable
+matrix-synapse-worker@<worker_name>.service`. For each `<worker_name>`, there
+should be a corresponding configuration file
+`/etc/matrix-synapse/workers/<worker_name>.yaml`.
+1. Start all the synapse processes with `systemctl start matrix-synapse.target`.
+1. Tell systemd to start synapse on boot with `systemctl enable matrix-synapse.target`/
+
+## Usage
+
+Once the services are correctly set up, you can use the following commands
+to manage your synapse installation:
+
+```sh
+# Restart Synapse master and all workers
+systemctl restart matrix-synapse.target
+
+# Stop Synapse and all workers
+systemctl stop matrix-synapse.target
+
+# Restart the master alone
+systemctl start matrix-synapse.service
+
+# Restart a specific worker (eg. federation_reader); the master is
+# unaffected by this.
+systemctl restart matrix-synapse-worker@federation_reader.service
+
+# Add a new worker (assuming all configs are set up already)
+systemctl enable matrix-synapse-worker@federation_writer.service
+systemctl restart matrix-synapse.target
+```