Add tool to convert Google Docs to Markdown

[Rob--W]

This tool was used to generate _minutes/2021-10-14-wecg.md in #104, and supersedes the manual process from #23.

To use this tool:

1. Open the HTML document in a web browser.
2. Select the text in Google Docs (https://docs.google.com/document/d/1QkwhEMtMS67JBUkl_WVPZ4lRSKoWcQNlLJSf_GwSXg8/edit) and paste this in the tool.
3. Click on the button to convert to markdown.
4. Copy-paste the generated markdown on a comment/issue box on Github to preview it (to check for correctness).
   (Note that the issue/comment field also linkifies @name-handles, but that does not happen in markdown previews in .md documents)

If the notes look fine, push to Github as follows:

5. Create the meeting-yyyy-mm-dd branch locally branched off origin/main, create a file called _minutes/yyyy-mm-dd-wecg.md and paste the markdown there. Also update the "Upcoming meetings" and "Past meetings" entries in _minutes/README.md. Commit the code to the branch and push the branch to the remote repo here.
6. The tool also shows a list of referenced issues. Create a pull request that mentions the listed issues and pull requests, optionally with commentary.

See #104 for the most recent example, or the pull requests linked from #23 for more comments.

[oliverdunk]

Took this for a spin and it worked out of the box!

[dotproto]

Just gave this a quick test and it looks great. Thanks for putting this together, @Rob--W!