feat: documentation site

Docusaurus 3 docs-only site at docsite/, served statically by nginx under
/docs on the marketing host. Covers getting started (self-hosted install
through first server and first licence), the control plane, Vantage HQ,
a reference section and operations.

Wired into docker-compose.site.yml as docsite (3005:80) and into the
image build workflow, rebuilding on its own directory only. Never added
to the self-hosted compose file.

docsite/static/img/favicon.svg

@@ -0,0 +1,12 @@
+<!--
+  The Vantage mark, traced from site/components/Logo.tsx. A favicon is an asset
+  rather than a component, and a browser tab has no access to the token block,
+  so the logo navy is a literal here — the same concession the email layout
+  makes. Keep it in step with --logo.
+-->
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="246 207 533 610">
+  <g transform="translate(0,1024) scale(0.1,-0.1)" fill="#0b2a58" stroke="none">
+    <path d="M4940 7767 c-96 -57 -528 -312 -960 -567 -678 -400 -1064 -628 -1187 -702 l-33 -20 0 -1357 0 -1357 293 -174 c160 -96 425 -252 587 -348 946 -559 1352 -799 1407 -833 34 -22 67 -39 72 -39 5 0 188 106 408 236 219 130 459 272 533 315 74 44 425 252 780 462 l645 382 0 1355 0 1355 -135 81 c-140 84 -1118 662 -1812 1070 -218 129 -402 236 -410 239 -7 3 -92 -42 -188 -98z m297 -331 c309 -182 971 -572 1208 -712 149 -88 372 -220 498 -294 l227 -135 0 -1175 0 -1175 -578 -342 c-317 -188 -694 -411 -837 -495 -143 -85 -344 -204 -447 -265 l-186 -111 -314 185 c-987 584 -1710 1013 -1725 1025 -10 8 -13 256 -13 1178 0 1099 1 1168 18 1182 9 8 150 93 312 188 162 96 417 246 565 333 805 476 1149 677 1156 677 4 0 56 -29 116 -64z" />
+    <path d="M3760 6109 c0 -6 187 -396 417 -867 229 -471 483 -994 564 -1162 l148 -305 232 0 233 0 271 560 c150 308 403 830 564 1160 160 329 291 605 291 612 0 19 -553 19 -568 1 -9 -12 -229 -480 -652 -1390 -73 -158 -136 -285 -140 -283 -3 2 -100 206 -215 452 -115 246 -291 624 -390 838 l-182 390 -287 3 c-202 2 -286 -1 -286 -9z" />
+  </g>
+</svg>