docs: Add a PLANKA Pro section to the README

Pro funds the Community edition, but the README mentioned it only in a single
line under How to Deploy. The new section sits between the demo and the key
features: what Pro adds, two buttons, four screenshots from 2.5.0, a
Community-versus-Pro table, and what an upgrade involves.

Every row of that table was checked against both codebases rather than taken
from the comparison matrix on the website, which carries cells that are
demonstrably wrong. The dangerous error is denying Community something it has,
because the reader has the opposite in front of them in their own instance —
so the stopwatch, webhooks, the REST API, custom fields, two-factor and the
notification providers are all marked as shared, and only what the code
actually gates is marked Pro. Where a claim could not be backed by the source
it was left out.

The upgrade paragraph names the releases the promote accepts as a snapshot
rather than as a standing fact, and points at the guide for the current list.
A hard-coded version window in the README of the repository whose version
keeps moving is wrong the moment the next minor ships, and nobody would
remember it was there.

The screenshots are referenced through raw.githubusercontent.com, so they
appear once this is pushed, the same way the logo and the demo already do.

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