mbproxy/docs: retire superseded design/plan docs and dissolve DL260/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phase tree were point-in-time planning artefacts now superseded by the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/src/Mbproxy/Admin/StatusHtmlRenderer.cs

@@ -5,7 +5,7 @@ namespace Mbproxy.Admin;
 /// <summary>
 /// Renders a <see cref="StatusResponse"/> as a self-contained HTML page.
 ///
-/// <para>Constraints (see <c>docs/design.md</c> status-page section):</para>
+/// <para>Constraints (see <c>docs/Operations/StatusPage.md</c>):</para>
 /// <list type="bullet">
 ///   <item>No external assets (CSS/JS/fonts/favicons) — firewalled networks only.</item>
 ///   <item><c>&lt;meta http-equiv="refresh" content="5"&gt;</c> for auto-refresh.</item>