feat(prs): add GH action to run builds on PRs into dev

[mkilp]

Hi everyone,

this is in response to @vimtor in #6291.

I called it a GH preview action, since I assume we want to eventually host a PR preview of the docs for easier review.
This PR creates an action that

• sets up all runtimes (go, docker buildx)
• installs dependencies
• runs a typecheck
• builds the platform, cli and docs

Maybe we can also set up the previews with the built-in SST cloudfront router for the docs eventually.

For now all this does is ensure a successful build.

LMK if you need any changes

cheers
marv