ci(*): add website deployment logic

[vdice]

• Adds website deployment logic (auto on pushes to main; support for manual/ad hoc triggering as well)
• Deploys to the prod version of the website on every push to main (once toggled below)
• Removes unecessary logic from dispatch.yml workflow (previously used to coordinate deploys)

As noted in the added deploy/README.md, the website here has been redirect-based for some time, pointing to the official docs sub-site at https://developer.fermyon.com/spin (all content being in the fermyon/developer repo). However, until we sunset the classic domain (spin.fermyon.dev), I'd like to have this auto-deployment logic available for any potential website changes in the meantime (including transitioning to a Fermyon Cloud app when ready). We can remove the works after the classic domain has been removed.

Deployment (to canary) from initial PR version tested in: https://github.com/fermyon/spin/actions/runs/4068116760

TODO:

• remove testing branch from deploy.yml
• update to deploy to prod on merges to main