docs: add a Sealos one-click deployment guide

## Proposal

Add a Sealos one-click deployment guide under `docs/docs/deployment/deployment-guides/` and link it from the existing deployment guide index.

The guide would use the same provider-page structure as the current Render, Nhost, DigitalOcean, Koyeb, and Flightcontrol guides.

## Deployment evidence

- Template: https://sealos.io/products/app-store/hasura
- Hasura GraphQL Engine: 2.49.5
- Data Connector Agent: 2.49.5
- PostgreSQL: 16.4
- Tested on Sealos: July 28, 2026
- Verified flow: required admin secret, Console access, automatic PostgreSQL source registration, table create and track, row insert and query, restart persistence, health checks, and zero-restart final run

## Scope

The PR will change documentation only:

- Add `docs/docs/deployment/deployment-guides/sealos.mdx`
- Add Sealos to the one-click and full deployment lists in `docs/docs/deployment/deployment-guides/index.mdx`

The Sealos template is maintained separately, so this repository keeps its existing runtime and installation manifests unchanged.

## Maintenance

I can maintain the Sealos deployment path and follow up when Hasura deployment requirements change.

Start with the existing Render, Nhost, DigitalOcean, Koyeb, and Flightcontrol guides under docs/docs/deployment/deployment-guides/ to follow their provider-page structure. Add docs/docs/deployment/deployment-guides/sealos.mdx using the verified Sealos flow, then update index.mdx in both deployment lists. Done means the guide is complete, linked from the index, and documents the tested deployment path.

Written by the indexing model from the issue text.