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/SUMMARY.md

@@ -0,0 +1,13 @@
+# Summary
+
+- [Introduction](introduction.md)
+- [Differences from upstream Conduit](differences.md)
+
+- [Example configuration](configuration.md)
+- [Deployment options](deploying.md)
+    - [Simple (Recommended)](deploying/simple.md)
+    - [Debian](deploying/debian.md)
+    - [Docker](deploying/docker.md)
+    - [Nix](deploying/nix.md)
+- [TURN](turn.md)
+- [Appservices](appservices.md)