docs(platform): low-level b3 API reference (#139)

Add a new "API Reference" section under docs/platform/api/ documenting the
ACE platform backend (b3) REST API at the endpoint level: HTTP method, path,
authentication, path/query parameters, and request/response schemas for every
operation, so a client can be implemented directly against it.

- 18 API groups (hld grouping) as folders, with per-subgroup pages and an
  index; 483 endpoints across 55 markdown files.
- GET endpoints were verified against a live deployment; each carries an inline
  verification note (status + reason for non-200s such as availability gating).
- Examples are sanitized (no secrets, hosts, emails, or tokens).

Signed-off-by: Tamal Saha <tamal@appscode.com>

Author: tamalsaha

api.html

@@ -0,0 +1,62 @@
+---
+layout: docs
+menu:
+  docsplatform_{{.version}}:
+    identifier: api
+    name: API Reference
+    weight: 40
+menu_name: docsplatform_{{.version}}
+section_menu_id: api
+---
+
+# ACE Platform API Reference
+
+Low-level reference for the AppsCode Cloud Engine (ACE) platform backend (`b3`)
+REST API. Every endpoint lists its HTTP method, path, authentication, path/query
+parameters, and request/response shapes so you can implement a client directly
+against it.
+
+All routes are served under the `/api/v1` prefix unless noted otherwise. The
+marketplace webhook service is a separate listener rooted at `/marketplace/api/v1`.
+
+## Authentication
+
+Most endpoints require a personal access token. Send it as an HTTP header:
+
+```
+Authorization: token <YOUR_TOKEN>
+```
+
+You can also pass it as a `token` or `access_token` query parameter. Token-management
+endpoints accept HTTP Basic auth. The web console uses a session cookie
+(`i_like_ace`); a session cookie alone does **not** authenticate the token-guarded
+REST endpoints — use a token.
+
+A machine-readable [OpenAPI 3.0.3 specification](../openapi.yaml) of this API is also
+available (`openapi.yaml`) — load it into any OpenAPI tool (Swagger UI, Redoc,
+`openapi-generator`) to explore the API or generate a client. A self-contained
+Swagger UI viewer (`api.html`, with the spec inlined) is available at the repository
+root.
+
+## API groups
+
+| Group | What it covers |
+|---|---|
+| [Identity: Users & Settings](../users-settings/) | Accounts, profile/security settings, tokens, credentials |
+| [Identity: Organizations & Teams](../organizations-teams/) | Orgs, members, teams, org tokens |
+| [Administration](../administration/) | Admin console, site settings |
+| [Authorization](../authorization/) | Custom roles & permissions |
+| [Cluster Management v1](../cluster-management-v1/) | Cluster lifecycle, Kubernetes proxy, Helm |
+| [Cluster Management v2](../cluster-management-v2/) | Hub-aware cluster API, subscriptions, gateways |
+| [Multi-cluster (OCM)](../multicluster-ocm/) | Hub/spoke, cluster sets, feature sets |
+| [Client Organizations](../client-organizations/) | Managed-service client orgs |
+| [Cloud Providers](../cloud-providers/) | Provider discovery for provisioning |
+| [ACE Installer](../ace-installer/) | Self-host installer bundles |
+| [ACE Upgrade](../ace-upgrade/) | Platform & cluster upgrades |
+| [Licensing & Contracts](../licensing-contracts/) | Contracts, licenses, registration |
+| [Billing Dashboard](../billing-dashboard/) | Usage reports & billing dashboards |
+| [Marketplace](../marketplace/) | Cloud-marketplace webhooks & metering |
+| [Monitoring & Telemetry](../monitoring-telemetry/) | Telemetry stack, Trickster auth proxy |
+| [Rancher Integration](../rancher/) | Rancher sync & proxy |
+| [Chart Repositories](../chart-repositories/) | Public Helm chart repositories |
+| [Miscellaneous](../miscellaneous/) | Version, markdown, health |

docs/platform/api/_index.md

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+---
+layout: docs
+menu:
+  docsplatform_{{.version}}:
+    identifier: api-ace-installer
+    name: ACE Installer
+    parent: api
+    weight: 100
+menu_name: docsplatform_{{.version}}
+section_menu_id: api
+---
+
+# ACE Installer API
+
+The ACE Installer API generates and manages self-host installer bundles for the
+AppsCode Container Engine (ACE) platform. It serves the installer options schema,
+generates and imports installer bundles, lists and inspects generated installers,
+and supports reconfigure/upgrade/versioning and marketplace-installer status.
+
+> **AppsCode-hosted only.** Every route under `/api/v1/ace-installer/...` is available
+> only on the AppsCode-hosted (SaaS) deployment. On self-hosted ACE installations these
+> routes are not registered and return `404 Not Found`. All calls require a bearer token,
+> an org context (resolved from the `org` query param), and per-action authorization
+> checks (`view_installers`, `create_installers`, `import_installers`,
+> `reconfigure_installers`, `upgrade_installers`, `download_installers`,
+> `delete_installers`).
+
+## Pages
+
+- [ACE Installer](../ace-installer.md) — schema/model, generate/import,
+  installer metadata and latest version, installers CRUD, reconfigure/upgrade,
+  versions/archives, and marketplace installer status.