mbproxy/docs: retire superseded design/plan docs and dissolve DL260/

The standalone design.md, kpi.md, operations.md, and the docs/plan/
phase tree were point-in-time planning artefacts now superseded by the
topic-organized docs/ tree (Architecture/, Features/, Operations/,
Reference/, Testing/). The DL260/ folder mixed a device-reference doc, a
test fixture, a sample test, and a screenshot; its contents now live in
their natural homes (dl205.md + mbtcp_settings.JPG under docs/Reference/,
dl205.json next to its launcher in tests/sim/, sample test dropped).

All cross-references in the surviving docs, README, CLAUDE.md, the config
template, and source comments are repointed to the new locations.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

mbproxy/docs/Architecture/ResponseCache.md

@@ -405,5 +405,3 @@ configuration described above.
   `mbproxy.cache.*` event catalogue with event IDs.
 - [`../Testing/Simulator.md`](../Testing/Simulator.md) — the
   `pymodbus` DL205 stand-in used by the end-to-end cache tests.
-- [`../design.md`](../design.md) — canonical design decisions and
-  rationale.