docs: add Curly to the maintained plugins table

[Foundasys]

Hi, this adds one row to the maintained plugins table: Curly, a plugin I build
and sell. It finds {placeholder} tags in a document and fills them from a
side panel, keeping formatting intact.

Being upfront: this is a commercial plugin (proprietary, with a free 14-day
trial), which is why I first asked in #11 whether commercial plugins fit this
table at all. That issue has been quiet for two weeks, so I figured a concrete
PR is easier to judge than an abstract question. @juliusknorr you set up this
table, so tagging you for the call. @rikled, you maintain the only plugin
listed so far, so your take is welcome too.

If the table is meant to stay FOSS-only, just say so and I'll close this, no
hard feelings. Feel free to close #11 with the same answer.

A few facts to make the review easy:

• Install page and download: https://curly.io/install/euro-office. Standard
  sdkjs-plugins layout, same server-side install route as MetaVox.
• Curly is also listed in the ONLYOFFICE plugin marketplace, the same catalog
  your install docs already point admins to, so it can be installed from there
  as well.
• Verified against the Euro-Office GA DocumentServer image (June 2026): tag
  detection and formatting-preserving replacement both work.
• Privacy: document content never leaves the editor. The plugin only calls our
  API for license and trial checks.
• Maintained by me (Foundasys, Netherlands), support via the site.

Dennis