Add advanced Context helpers guide

The manual had no single place where readers could find all the
advanced Context methods.  parseUri(), routeActivity(),
getSignedKey()/getSignedKeyOwner(), getDocumentLoader(),
getActorKeyPairs(), lookupObject()/lookupWebFinger()/lookupNodeInfo(),
traverseCollection(), and RequestContext.request/url were scattered
across the inbox, access-control, webfinger, and nodeinfo pages.

This commit adds docs/manual/context-advanced.md, a dedicated guide
that gathers every advanced helper in one place with usage examples,
a quick-reference table, and links back to the protocol-specific pages
for deeper coverage.  The existing Context page now carries a pointer
to the new guide.

Closes #670

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dahlia

.zed/settings.json

@@ -31,6 +31,23 @@
         "..."
       ]
     },
+    "Markdown": {
+      "formatter": {
+        "external": {
+          "command": "hongdown",
+          "arguments": [
+            "-"
+          ]
+        }
+      },
+      "language_servers": [
+        "!deno",
+        "!typescript-language-server",
+        "!vtsls",
+        "!biome",
+        "..."
+      ]
+    },
     "TypeScript": {
       "code_actions_on_format": {
         "source.sortImports": true