Add Astro TypeScript template

[leovoon]

Summary

• add a new templates/astro-ts template based on the existing Astro + SpacetimeDB integration work
• include Astro SSR, a React client island for realtime updates, and a small server:defer example
• add Astro quickstart documentation under docs/docs/00100-intro/00200-quickstarts/00152-astro.md

Validation

• pnpm install
• pnpm -F spacetimedb build
• pnpm -F ./templates/astro-ts build

Notes

• I did not add astro-ts to the built-in template listing in spacetime dev docs, to match the current nextjs-ts pattern.

[CLAassistant]

All committers have signed the CLA.

[cloutiertyler]

Very clean PR!