Add Gandi DNS docs and rewrite homepage intro (#115)

## Summary
- New reference card (`docs/reference/infrastructure/gandi.md`) covering DNS records, Pulumi config, TLS integration
- New how-to guide (`docs/how-to/gandi-operations.md`) for DNS deployment and PAT cycling with `pbpaste` shortcut
- Rewritten homepage intro for wider audience ahead of public docs.eblu.me
- Cross-linked from reference index, routing, caddy, and how-to index
- Fixed PAT expiration inaccuracy in `pulumi/gandi/README.md` (max is 90 days, not 30)

## Test plan
- [ ] Verify wiki-links resolve in Quartz build
- [ ] Review gandi reference card for accuracy
- [ ] Review gandi-operations how-to for accuracy
- [ ] Check homepage reads well for external visitors

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/115

docs/how-to/how-to.md

@@ -20,6 +20,7 @@ Task-oriented instructions for common BlumeOps operations. These guides assume y
 | Guide | Description |
 |-------|-------------|
 | [[update-tailscale-acls]] | Update Tailscale access control policies |
+| [[gandi-operations]] | Manage DNS records and cycle the Gandi API token |
 | [[use-pypi-proxy]] | Configure pip and publish packages to devpi |
 
 ## Documentation