Add Plone documentation to sites that attract first-time contributors

[stevepiercy]

This is a pilot program to attract first-time contributors to Plone through its documentation. It's following the model established by icalendar in its first-time contributor documentation.

• I submitted a PR to Up for Grabs at Add plone-documentation.yml up-for-grabs/up-for-grabs.net#5586. See example listing for icalendar.
• I applied to Good First Issue. Awaiting decision. See sample listing for icalendar.
• Following https://hacktoberfest.com/participation/#maintainers, I've add the topic "hacktoberfest" to the repo.
• I've added the following labels to work with these programs, and applied the labels to become eligible.
  • 99 tag: good first issue
  • 99 tag: spam
  • 99 tag: hacktoberfest
  • 99 tag: hacktoberfest-accepted
  • 99 tag: invalid
• Update documentation to reflect this pilot program.
• Also consider adding low or non-code contributions, including translations, technical blog posts or tutorials, case studies, graphic design, and other community building tasks.

[stevepiercy]

We've been accepted by Up For Grabs. https://up-for-grabs.net/#/filters?tags=&names=836 🥳

[stevepiercy]

I'm not sure what's up with GFI. I started a discussion, and I might follow up with an issue, if there's no response.

DeepSourceCorp/good-first-issue#2624