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>

.gitlab/route-map.yml

@@ -0,0 +1,3 @@
+# Docs: Map markdown to html files
+- source: /docs/(.+)\.md/
+  public: '\1.html'