Update documentation

- [x] The div / button example is old and should be updated in "The Basics"
- [x] Nested selectors examples should include the `@code` section
- [ ] Ensure `@@` as needed in the examples
- [ ] Add a page for tag syntax

Do a check of all other pages.

Start by reviewing the "The Basics" documentation and the existing nested-selector examples, then check the other pages as requested. Update the marked examples, ensure `@@` is used where needed, and add a page for tag syntax; the work is done when all listed documentation tasks and the broader page check are complete.

Written by the indexing model from the issue text.