[Docs] Add JSON-LD and llms.txt for AI search (#4458)

Two SEO changes for the docs site. Nothing changes the rendered page.

- **JSON-LD**: `Organization` with the same `@id` as swmansion.com, plus
`SoftwareSourceCode` for the library, derived from `siteConfig`. The
identical `@id` is what makes engines read the docs and the company site
as one entity instead of two unrelated ones.
- **`llms.txt`**, generated at build — no new dependency. Lists every
docs page. The docs build now fails if the file is missing or lists
nothing.

Both come from one local plugin, `plugins/swm-geo.js`, registered with a
single line. The same change is going out across the docs repos;
reference: software-mansion/react-native-reanimated#10332.

**Check:** CI prints `llms.txt lists N pages`.