create an explicit (& independent) 6.4 docs build

[heckj]

Set up configuration and build for a /6.4/ hosting base path and combined documentation

currently that's being built as /latest/ - but we'll want a /6.4/ copy (with relevant canonical links in HTML) for when the next Swift release is starting to hit the ground

[finagolfin]

For platforms like Android or Embedded, that host their own docs in their examples repos, do we need to set up 6.4 branches to start versioning our docs? Let us know what you want to do there.