Add table of contents to documentation

It will be helpful to the reader to be able to get an overview of the documentation content and quickly navigate to the
section of interest.

The table of contents are automatically generated using the markdown-toc tool.

Because it can be easy to forget to update the table of contents when the documentation content is changed, I have added
a CI workflow to check for missed updates to readme ToC. On every push or pull request that affects the repository's
documentation, it will check whether the table of contents matches the content.

.github/workflows/check-toc.yml

@@ -0,0 +1,58 @@
+name: Check ToC
+
+# See: https://docs.github.com/en/free-pro-team@latest/actions/reference/events-that-trigger-workflows
+on:
+  push:
+    paths:
+      - ".github/workflows/check-toc.yml"
+      - "README.md"
+      - "FAQ.md"
+  pull_request:
+    paths:
+      - ".github/workflows/check-toc.yml"
+      - "README.md"
+      - "FAQ.md"
+  workflow_dispatch:
+  repository_dispatch:
+
+jobs:
+  check:
+    name: ${{ matrix.file.name }}
+    runs-on: ubuntu-latest
+
+    strategy:
+      fail-fast: false
+
+      matrix:
+        file:
+          - name: README.md
+            # Max ToC depth, for use with the markdown-toc --maxdepth flag.
+            maxdepth: 4
+          - name: FAQ.md
+            maxdepth: 3
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v2
+
+      - name: Get week number for use in cache key
+        id: get-date
+        run: |
+          echo "::set-output name=week-number::$(date --utc '+%V')"
+
+      - name: Load dependencies cache
+        uses: actions/cache@v2
+        with:
+          path: ~/.npm
+          key: ${{ runner.os }}-node-markdown-toc-${{ steps.get-date.outputs.week-number }}
+          restore-keys: |
+            ${{ runner.os }}-node-markdown-toc-
+
+      - name: Install markdown-toc
+        run: sudo npm install --global markdown-toc
+
+      - name: Rebuild ToC
+        run: markdown-toc --bullets=- --maxdepth=${{ matrix.file.maxdepth }} -i "${{ github.workspace }}/${{ matrix.file.name }}"
+
+      - name: Check ToC
+        run: git diff --color --exit-code