docs: rewrite README — production/VC-ready, logo + v0.5.5 feature set

Replaces the stale v0.4.1 README with a polished, accurate overview:
centered brand-mark header + tagline + badges, a "Why OpenPXE" pitch,
a scannable Highlights section, and a "Built in Rust" section framed on
real properties (single ~18MB static musl binary, no GC, async Tokio,
workspace-wide unsafe deny, OpenSSL-free pure-Rust crypto, sub-minute
zigbuild images).

Surfaces everything shipped since v0.4.1: SMB + NFS + SFTP remote ISO
libraries (with a comparison table), SAML SSO, branding, notifications,
unattended installs, per-MAC host bindings, and layered figment config.
Quick-start, env table, OpenShift, and health/observability all updated
to v0.5.5. Adds docs/openpxe-logo.svg (render-safe static copy of the
web-UI mark) for the header.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

docs/openpxe-logo.svg

@@ -0,0 +1,21 @@
+<svg viewBox="0 0 24 24" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="OpenPXE">
+  <title>OpenPXE</title>
+  <!-- Static README mark: the "rainbow-horizon" medallion from the web UI,
+       with the SMIL animation removed so it renders reliably as an <img>
+       on Gitea/GitHub. -->
+  <defs>
+    <linearGradient id="opxRainbow" x1="0" y1="0" x2="1" y2="0">
+      <stop offset="0%"     stop-color="#330f1f"/>
+      <stop offset="12.56%" stop-color="#c83228"/>
+      <stop offset="25.06%" stop-color="#fb8841"/>
+      <stop offset="37.56%" stop-color="#d3dd92"/>
+      <stop offset="50.06%" stop-color="#59824f"/>
+      <stop offset="62.06%" stop-color="#002414"/>
+      <stop offset="74.06%" stop-color="#00143d"/>
+      <stop offset="86.06%" stop-color="#2874d7"/>
+      <stop offset="100%"   stop-color="#99c2ff"/>
+    </linearGradient>
+  </defs>
+  <circle cx="12" cy="12" r="10.5" fill="url(#opxRainbow)"
+          stroke="rgba(0,0,0,0.18)" stroke-width="0.6"/>
+</svg>