Add MLC for link checking (#1097)

* docs: add mlc config and github action to check links

Ignores relative links since we can only check those from the main
website project.

* docs: update redirecting links and configure some ignore patterns

.github/workflows/mlc-link-check.yaml

@@ -0,0 +1,17 @@
+name: Check links
+on:
+  pull_request:
+    types: [opened, synchronize, reopened]
+  push:
+    branches:
+      - main
+jobs:
+  linkcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v2
+      - name: Markup Link Checker (mlc)
+        uses: becheran/mlc@v0.16.1
+        with:
+          args: ./docs