docs: build docs using mdBook, build in CI, deploy to gitlab pages

squashed from https://gitlab.com/famedly/conduit/-/merge_requests/604

added differences.md

Co-authored-by: Charles Hall <charles@computer.surgery>
Co-authored-by: strawberry <strawberry@puppygock.gay>
Signed-off-by: strawberry <strawberry@puppygock.gay>

docs/appservices.md

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+# Setting up Appservices
+
+## Getting help
+
+If you run into any problems while setting up an Appservice, write an email to `timo@koesters.xyz`, ask us in [#conduit:fachschaften.org](https://matrix.to/#/#conduit:fachschaften.org) or [open an issue on GitLab](https://gitlab.com/famedly/conduit/-/issues/new).
+
+## Set up the appservice - general instructions
+
+Follow whatever instructions are given by the appservice. This usually includes
+downloading, changing its config (setting domain, homeserver url, port etc.)
+and later starting it.
+
+At some point the appservice guide should ask you to add a registration yaml
+file to the homeserver. In Synapse you would do this by adding the path to the
+homeserver.yaml, but in Conduit you can do this from within Matrix:
+
+First, go into the #admins room of your homeserver. The first person that
+registered on the homeserver automatically joins it. Then send a message into
+the room like this:
+
+    @conduit:your.server.name: register-appservice
+    ```
+    paste
+    the
+    contents
+    of
+    the
+    yaml
+    registration
+    here
+    ```
+
+You can confirm it worked by sending a message like this:
+`@conduit:your.server.name: list-appservices`
+
+The @conduit bot should answer with `Appservices (1): your-bridge`
+
+Then you are done. Conduit will send messages to the appservices and the
+appservice can send requests to the homeserver. You don't need to restart
+Conduit, but if it doesn't work, restarting while the appservice is running
+could help.
+
+## Appservice-specific instructions
+
+### Remove an appservice
+
+To remove an appservice go to your admin room and execute
+
+`@conduit:your.server.name: unregister-appservice <name>`
+
+where `<name>` one of the output of `list-appservices`.
+
+### Tested appservices
+
+These appservices have been tested and work with Conduit without any extra steps:
+
+- [matrix-appservice-discord](https://github.com/Half-Shot/matrix-appservice-discord)
+- [mautrix-hangouts](https://github.com/mautrix/hangouts/)
+- [mautrix-telegram](https://github.com/mautrix/telegram/)
+- [mautrix-signal](https://github.com/mautrix/signal/) from version `0.2.2` forward.
+- [heisenbridge](https://github.com/hifi/heisenbridge/)