docs: add extensions reference list and update navigation

- Add Developer-Guide_Extensions-List.md: alphabetical reference of
  all 60+ official extensions with short descriptions
- Rename 'Extensions' nav entry to 'Extensions Framework' to better
  reflect the page content (framework concepts, not extension catalog)
- Fix trailing whitespace on Netconfig nav entry

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: iav