Add Plain CMS to the CMS section

[victorantos]

Adding Plain CMS (https://github.com/plain-cms/plain) to the CMS section.

Why it fits here: the section already lists Decap CMS, Prose and Coisas, and Plain CMS is in the same Git-based family — content is Markdown files plus JSON config committed to the repo. The one thing it does differently from those neighbours is that it contains the static site generator as well, so there is no separate SSG to wire up. The build emits HTML pages and a read-only JSON API at /api/site.json.

Facts: MIT, v1.13, about 2,500 lines of vanilla JavaScript with one runtime dependency (marked). No database, no server, no hosted service. Deploys to GitHub Pages or Cloudflare Pages.

Disclosure: I wrote it, so weigh this accordingly. It is early — 13 stars and one maintainer. If the bar for this section is higher than "actively maintained and genuinely Jamstack", tell me and I will close this myself.

Checklist:

• One suggestion in this PR.
• Added to the bottom of the section.
• Description starts with a capital letter and ends with a period.
• No trailing whitespace.
• Searched the README for duplicates first.