This allows URLs to be passed through to the server (where they are
properly redirected), instead of serving `index.html` from the SW.
Known issue:
`/docs/` will be passed through to the server. `/docs` (without the
trailing slash) will be correctly treated as a navigation URL and
handled by the SW.
We don't link to `/docs/` from within the app, but if there are external
links to `/docs/` they will require a round-trip to the server and will
not work in offline mode.
PR Close#19795