feat(docs): upgrade fully and expose SDK docgen

- migrate from Astro 4.x to 5.x
- migrate from Starlight ~0.2 to ~0.3
- migrate from `tailwindcss` 3.x to 4.x
- upgrade `vite` from 5.x to 6.x
- upgrade `@graphiql/plugin-explorer` from 3.x to 5.x
- build and render SDK documentation per-package
- add custom plugin for rendering SDK examples
- enable expressive code for rich examples

Signed-off-by: Eric Hegnes <eric@hegnes.com>