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/docs/index.md

@@ -0,0 +1,37 @@
+---
+id: index
+title: Vantage documentation
+sidebar_label: Overview
+slug: /
+---
+
+# Vantage documentation
+
+Vantage is a self-hosted, multi-tenant infrastructure control plane. It manages
+SSH keys, runs scripted workflows, watches services, stores secrets, opens
+browser consoles and applies OS updates across a fleet of servers.
+
+A central server drives a lightweight agent installed on each managed machine.
+The agent connects **outbound only**, so managed servers need no inbound
+firewall holes.
+
+## Where to start
+
+| If you want to | Read |
+| --- | --- |
+| Understand what the pieces are | [What is Vantage](./getting-started/what-is-vantage.md) |
+| Run it on your own hardware | [Self-hosted install](./getting-started/self-hosted-install.md) |
+| Enrol your first machine | [Add your first server](./getting-started/first-server.md) |
+| Manage your account, licence or billing | [Vantage HQ](./hq/accounts-and-signup.md) |
+| Look something up | [Reference](./reference/environment-variables.md) |
+
+## The two products
+
+**Vantage** is the control plane — the thing you sign in to in order to manage
+servers. It runs either on your own infrastructure or as a cloud instance we
+run for you.
+
+**Vantage HQ** is the portal at `vantage-hq.hostxtra.co.uk` where you manage the
+account behind those instances: who is on your team, which instances exist, what
+licence each one holds and how it is billed. HQ never manages your servers, and
+a Vantage instance never depends on HQ being reachable in order to run.