This PR removes the `packages/payload/src/assets` folder for the
following reasons:
- they were published to npm. Removing this decreases the install size
of payload (excluding dependencies) from 6.22MB => 5.12MB
- most assets were unused. The only used ones were moved to a different
directory that does not get published to npm
This also updates some outdated asset URLs in our examples
bb17cc3ea8
