docs: align VitePress site with REST send, optional gateway and slash commands

- deployment: drop DISCORD_CHANNEL_ID, add ADMIN_USER_IDS, PKCS#8 key note,
  applications.commands invite scope, and an optional Gateway section
- getting-started: PKCS#8 .dev.vars example, remove channel ID
- introduction: REST delivery, optional DiscordGateway DO, delivery dedup,
  /admin Web UI in architecture and data flow
- index: refresh Workers feature, add Web UI & slash commands feature

docs/index.md

@@ -19,9 +19,9 @@ features:
   - title: Flexible Filtering
     details: Filter by event type, repo, actor, action, branch (including PRs), and keyword (with regex support). Exclude patterns with a flag.
   - title: Cloudflare Workers
-    details: Runs on Cloudflare's edge network with Durable Objects for persistent Discord Gateway connections and KV for storage.
-  - title: OAuth & User Actions
-    details: "GitHub App OAuth flow enables user-initiated actions: comment on issues, merge PRs, add reactions — all via Bearer token auth."
+    details: Runs on Cloudflare's edge network. Sends via the Discord REST API, with an optional Durable Object Gateway connection for online status and slash commands.
+  - title: Web UI & Slash Commands
+    details: "Manage routes and view send logs from a built-in admin console. Link your GitHub account and comment on issues/PRs as yourself via /gh commands."
   - title: Signature Verification
     details: HMAC-SHA256 webhook signature verification using the Web Crypto API with timing-safe comparison.
   - title: Graceful Degradation