tidy up docs publishing flow

[vito]

these are tricky problems, so the devil will be in the details, but here are some rough goals:

• newly added stdlib bindings should be annotated with the version they were added in
  • this can be done like ^{:added-in "v0.1.1"}
  • might even be OK to let these be published ahead of release
• docs should be auto-built and pushed to a different branch
  • it's annoying having the noise in PRs
  • docs changes for releases can live in a PR and be merged at release time
• built docs should live in another repo?
  • generated output inflates repo size, this will add up over time