docs: add git providers and PR Insights pages for Coder Agents (#24447)

Adds two new documentation pages under platform controls for Coder
Agents:

- **Git Providers** (`git-providers.md`) — documents the `API_BASE_URL`
configuration required for self-hosted GitHub Enterprise deployments.
Positions it as an extension of the existing [external
auth](https://coder.com/docs/admin/external-auth) setup to support Coder
Agents features that need richer git host API access: the in-chat diff
viewer and PR Insights.
- **PR Insights** (`pr-insights.md`) — documents the PR analytics
dashboard, requirements for PR data to appear, and troubleshooting.
Links to git-providers for GHE setup.

Also updates the platform controls index and docs manifest.

---

> PR generated with Coder Agents

docs/manifest.json

@@ -1251,6 +1251,18 @@
 									"path": "./ai-coder/agents/platform-controls/usage-insights.md",
 									"state": ["early access"]
 								},
+								{
+									"title": "Git Providers",
+									"description": "Git provider configuration for the diff viewer and PR Insights",
+									"path": "./ai-coder/agents/platform-controls/git-providers.md",
+									"state": ["early access"]
+								},
+								{
+									"title": "PR Insights",
+									"description": "Pull request analytics for Coder Agents",
+									"path": "./ai-coder/agents/platform-controls/pr-insights.md",
+									"state": ["early access"]
+								},
 								{
 									"title": "Data Retention",
 									"description": "Automatic cleanup of old conversation data",