docs(router): add How To section to docs navigation sidebar
The how-to/ directory contains 20+ step-by-step guides covering
authentication, search params, testing, SSR, UI integrations, and more,
but none of them were linked from the docs navigation config.
Add a new 'How To' section to docs/router/config.json between
'Guides' and 'API' with entries for all existing how-to guides.
Fixes #6801