Use Github Actions to automatically deploy documentation

.cargo/config

@@ -1,2 +1,5 @@
 [alias]
 xtask = "run --package xtask --"
+
+[doc.extern-map.registries]
+crates-io = "https://docs.rs/"

.github/workflows/docs.yaml

@@ -0,0 +1,29 @@
+on:
+  push:
+    branches: [main]
+
+jobs:
+  build-deploy:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout repo
+      uses: actions/checkout@v2
+    - name: Install nightly rust toolchain
+      uses: actions-rs/toolchain@v1
+      with:
+        profile: minimal
+        # Needed for use of unstable options
+        toolchain: nightly
+        override: true
+    - name: Build docs
+      uses: actions-rs/cargo@v1
+      env:
+       RUSTDOCFLAGS: "--enable-index-page -Zunstable-options"
+      with:
+        command: doc
+        args: --no-deps --workspace --exclude xtask --features api,events,signatures,appservice-api,client-api,federation-api,identity-service-api,push-gateway-api,either,rand,markdown,compat,unstable-pre-spec -Zrustdoc-map
+    - name: Deploy to docs branch
+      uses: JamesIves/github-pages-deploy-action@4.1.0
+      with:
+        branch: docs
+        folder: target/doc