Add doc coverage badge

[jzombie]

Here's an example of how to do it:

https://github.com/bewee/rustdoc-coverage-action-example/blob/main/.github/workflows/doc-coverage-badge.yml

name: Doc Coverage Badge

on:
  push:
    branches:
      - main

jobs:
  badge:
    name: Doc Coverage Badge
    runs-on: ubuntu-latest
    steps:
      - name: Checkout
        uses: actions/checkout@v2
      - name: Install rust toolchain
        uses: actions-rs/toolchain@v1
        with:
          toolchain: nightly
      - name: Calculate doc coverage
        id: coverage
        uses: bewee/rustdoc-coverage-action@v1
        with:
          percentage-format: 0%
      - name: Create Awesome Badge
        uses: schneegans/dynamic-badges-action@v1.1.0
        with:
          auth: ${{ secrets.GIST_SECRET }}
          gistID: 7af9a0b9ffd24658ce9964baf703d802
          filename: doc-coverage.json
          label: doc cov
          message: ${{ steps.coverage.outputs.documented }}
          color: orange