Add Mastodon installation docs

docs/administration/mastodon.md

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+# Installing Soapbox over Mastodon
+
+It is possible to run Soapbox as your main frontend on top of Mastodon.
+This will replace the homepage and all static pages with Soapbox, using Mastodon only as the API.
+
+To do so, shell into your server and unpack Soapbox:
+
+```sh
+mkdir -p /opt/soapbox
+
+curl -L https://gitlab.com/soapbox-pub/soapbox-fe/-/jobs/artifacts/develop/download?job=build-production -o soapbox-fe.zip
+
+busybox unzip soapbox-fe.zip -o -d /opt/soapbox
+```
+
+Now create an Nginx file for Soapbox with Mastodon.
+If you already have one, replace it:
+
+```sh
+curl https://gitlab.com/soapbox-pub/soapbox-fe/-/raw/develop/installation/mastodon.conf > /etc/nginx/sites-available/mastodon
+```
+
+Edit this file and replace all occurrences of `example.com` with your domain name.
+Uncomment the SSL lines if you've enabled SSL, otherwise do that first.
+
+Finally, ensure the file is symlinked, then restart Nginx:
+
+```sh
+ln -s /etc/nginx/sites-available/mastodon /etc/nginx/sites-enabled/mastodon
+
+systemctl restart nginx
+```
+
+If all is well, hopefully this worked!
+If not, run `nginx -t` to see if anything is amiss, and try reviewing Mastodon's [install guide](https://docs.joinmastodon.org/admin/install/).