Docs: Explain why Layouts (RSCs) can't access `pathname` (#64785)
- Add section explaining why `pathname` can't be accessed in a layout
([file](https://github.com/vercel/next.js/pull/64785/files#diff-8eb7b23d3da342a1a407cdb6cc9c2d0dc2a455091ba2c4b6dbc41ba6cf840367))
- Split pages and layouts files (to add layout-specific examples in the
future)
- Clean up the existing active links example
I'll add layout-specific examples in a follow-up PR, after creating some
demos.