Split README into a bunch of files in docs/

docs/configuring-playbook-own-webserver.md

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+# Using your own webserver, instead of this playbook's nginx proxy (optional)
+
+By default, this playbook installs its own nginx webserver (in a Docker container) which listens on ports 80 and 443.
+If that's alright, you can skip this.
+
+If you don't want this playbook's nginx webserver to take over your server's 80/443 ports like that,
+and you'd like to use your own webserver (be it nginx, Apache, Varnish Cache, etc.), you can.
+
+All it takes is editing your configuration file (`inventory/matrix.<your-domain>/vars.yml`):
+
+```
+matrix_nginx_proxy_enabled: false
+```
+
+**Note**: even if you do this, in order [to install](#installing), this playbook still expects port 80 to be available. **Please manually stop your other webserver while installing**. You can start it back again afterwards.
+
+**If your own webserver is nginx**, you can most likely directly use the config files installed by this playbook at: `/matrix/nginx-proxy/conf.d`. Just include them in your `nginx.conf` like this: `include /matrix/nginx-proxy/conf.d/*.conf;`
+
+**If your own webserver is not nginx**, you can still take a look at the sample files in `/matrix/nginx-proxy/conf.d`, and:
+
+- ensure you set up (separate) vhosts that proxy for both Riot (`localhost:8765`) and Matrix Synapse (`localhost:8008`)
+
+- ensure that the `/.well-known/acme-challenge` location for each "port=80 vhost" is an alias to the `/matrix/ssl/run/acme-challenge` directory (for automated SSL renewal to work)
+
+- ensure that you restart/reload your webserver once in a while, so that renewed SSL certificates would take effect (once a month should be enough)