docs: introduce event handlers utility section (#388)

* docs: adjust navigation to allow future handlers

Signed-off-by: heitorlessa <lessa@amazon.co.uk>

* chore: add back to top button feature

Signed-off-by: heitorlessa <lessa@amazon.co.uk>

* chore: add highlight for inline block

Signed-off-by: heitorlessa <lessa@amazon.co.uk>

* docs: add initial structure

Signed-off-by: heitorlessa <lessa@amazon.co.uk>

* docs(appsync_handler): polish advanced section

* docs(appsync_handler): add nested mappings & async

* docs(appsync_handler): add last touches before review

* docs(appsync_handler): add ltesting section

Author: heitorlessa