Automate link to ReadTheDocs PR preview

References

helping enable testing on #633

Problem

Finding the correct link to the ReadTheDocs preview is hidden inside the Checks UI, and attention is only drawn to it if it fails.

Proposed Solution

Add a workflow that posts a PR comment with a badge. Bonus: have the badge update with the status of the build.

Additional context

name: RTD Preview

on:
  pull_request_target:
    types: [opened]

permissions:
  pull-requests: write

jobs:
  binder:
    runs-on: ubuntu-latest
    steps:
      - name: Comment on the PR with the RTD preview
        uses: actions/github-script@v6
        with:
          github-token: ${{ secrets.GITHUB_TOKEN }}
          script: |
            var PR_NUMBER = context.issue.number
            github.rest.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: `[![readthedocs](NEED-A-BADGE.svg)](https://lumino--${PR_NUMBER}.org.readthedocs.build/en/${PR_NUMBER}) :point_left: Try it on ReadTheDocs`
            })

Ideally, there would be an RTD status badge for this specific PR, but this isn't supported yet. A sorta-ghetto approach would be to summon the logo, a la:

![the logo](https://lumino--633.org.readthedocs.build/en/633/_static/jupyter_logo.svg "the docs built once")

yielding the logo

This would be broken if the docs never built once, but then there would be little to distinguish it if a successive build broke it. But github caches all this stuff, so maybe it's not worth it if it ever looks broken when its not.

jupyterlab / lumino