README: a short overview that points into docs/

It described the layout from before 0.4.0 (items across the top, a
player below), and carried long sections on OPML, the log view and
this server's own deploy steps, all of which docs/ and CLAUDE.md cover.
It now says what ipx does, how to run it with Docker or from source,
the first sign-in, the TLS caveat, where the docs are, and the tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0173mGu6rK18Ne7UGTwAaVJV

CHANGELOG.md

@@ -10,6 +10,11 @@ The long form, with what was wrong before and how it was found, is in
 
 ## [Unreleased]
 
+### Changed
+
+- The README is a short overview of what ipx does and how to run it, and points into `docs/` for
+  the rest. It still described the layout from before 0.4.0.
+
 ## [0.4.0] - 2026-09-11
 
 ### Added