docs: improve docs and agent discovery metadata

[clockwork-labs-bot]

Summary

• Fix TypeScript view examples to use ctx.sender as a property, matching the server SDK ViewCtx API.
• Update the architecture overview TypeScript view snippet to use players.rowType and undefined for optional view returns.
• Clarify shared ViewContext prose so it does not imply every language uses a callable ctx.sender() API.
• Improve docs agent-readiness metadata:
  • publish /docs/robots.txt with a docs sitemap directive and Content-Signal policy
  • add Markdown alternate links for the existing /docs/llms.txt and /docs/llms-full.txt outputs
  • generate /.well-known/agent-skills-style discovery metadata under /docs/.well-known/agent-skills/ from the repo's existing skills/ source files during docs builds

Validation

• pnpm --dir docs build
• pnpm --dir docs typecheck
• Verified the docs build emits robots.txt and .well-known/agent-skills/index.json.
• Verified generated Agent Skills SHA-256 digests match the emitted SKILL.md artifacts.

Notes

The Cloudflare agent-readiness scan for spacetimedb.com still depends on the root/marketing host exposing root-level files such as /robots.txt, /sitemap.xml, and /.well-known/agent-skills/index.json. This PR makes the docs origin produce the corresponding docs-scoped artifacts at /docs/...; the root host can route or mirror these if we want the exact spacetimedb.com scan to pick them up.