GitHub markdown -> Plane Pages sync with public API support

[marinosabijan]

We need first-class syncing from GitHub markdown files into Plane Pages.

Current problem:

• Pages are the natural target for docs, design systems, reference content, runbooks, and release notes.
• In practice, the public/API-key surface appears insufficient for unattended page management.
• That forces automation users to either rely on unsupported browser/session workarounds or use work items as a docs substitute.

Requested capability:

1. Sync markdown files from a GitHub repo into Plane Pages.
2. Map files to stable Plane pages by repo path or external ID.
3. Support create, update, list, and archive/delete through the public/API-key surface.
4. Support folder -> nested page hierarchy.
5. Support one index/root page plus child pages.
6. Preserve page identity and URL across updates.
7. Optionally expose sync status, last synced commit SHA, and source link.

Why this matters:

• design systems
• product docs
• runbooks
• release notes
• board/company knowledge bases
• operational manuals

Real-world example:
We just had to build repo -> Plane automation using work items because they support stable API-key mutation, even though Pages are the correct product surface for this kind of content. A native GitHub markdown -> Pages sync would be a much better solution.

The key ask here is not just page CRUD. It is a docs-native automation path that makes Plane Pages a durable destination for repository-backed knowledge.