Set 'About' homepage URL to the Fern docs site once deployed

[resker]

Summary

The repo's "About" sidebar does not currently surface a docs URL. The "About" sidebar should link to the canonical documentation site — this is a standard discoverability improvement for GitHub repos.

Current state:
```
$ gh api repos/NVIDIA/topograph --jq .homepage
(empty)
```

Dependency

Blocked on #249 — the Fern documentation scaffolding PR by @pdmack. Once that merges and the publish-fern-docs.yml workflow runs its first deploy, the Fern site will have a stable URL.

Until then, setting the homepage to a README anchor or /tree/main/docs is strictly worse than what Fern will provide, so this is best left until after #249 lands.

How (after #249 deploys)

Requires admin on the repo. Once the Fern URL is known:

```bash
gh repo edit NVIDIA/topograph --homepage
```

Or via GitHub UI: repo page → gear icon next to "About" → Website field.

Reference

• PR #249 — Fern documentation scaffolding (by @pdmack)
• Example: NVIDIA/TensorRT-LLM sets its homepage to https://nvidia.github.io/TensorRT-LLM

[resker]

@dmitsh - it looks like this is now successfully publishing to: https://docs.nvidia.com/topograph