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>

flake.nix

@@ -208,6 +208,35 @@
       packages = {
         default = package pkgsHost;
         oci-image = mkOciImage pkgsHost self.packages.${system}.default;
+
+        book =
+          let
+            package = self.packages.${system}.default;
+          in
+          pkgsHost.stdenv.mkDerivation {
+            pname = "${package.pname}-book";
+            version = package.version;
+
+            src = nix-filter {
+              root = ./.;
+              include = [
+                "book.toml"
+                "conduit-example.toml"
+                "README.md"
+                "debian/README.md"
+                "docs"
+              ];
+            };
+
+            nativeBuildInputs = (with pkgsHost; [
+              mdbook
+            ]);
+
+            buildPhase = ''
+              mdbook build
+              mv public $out
+            '';
+          };
       }
       //
       builtins.listToAttrs
@@ -276,6 +305,9 @@
 
           # Needed for our script for Complement
           jq
+
+          # Needed for finding broken markdown links
+          lychee
         ]);
       };
     });