remove legacy docs (#9236)

with the documentation content migrated, remove the legacy content to
preserve a single source of truth

### Motivation:

The documentation content from the `Documentation` directory is now
primarily contained in `Sources/PackageManagerDocs` - and we should have
a clear, single source of truth. This removes the legacy
markdown content in favor of the new location (where migrated - some
older pieces linger), and points any
interested parties to the new location.

### Modifications:

Remove legacy documentation in `documentation` that's been replicated
elsewhere
Update README to reference hosted location at swift.org
Update Documentation/README to reference new location for content

### Result:

Consolidated documentation

Author: heckj