feat: add to IPFS on PR (filecoin-project#976)

* feat: add to IPFS on PR

- Adds work flow to add the rendered website to IPFS, whenever a commit is pushed. Adds a github status on the commit with a link to the rendered site.
- Replaces hardcoded paths with the $GITHUB_WORKSPACE var. This makes the build work on forked repos, where the repo has to be renamed, and shields us from future issues if actions change their workspace dir.
- Updated IPFS github action to pin to work with the new style github actions https://github.com/ipfs-shipyard/ipfs-github-action


License: MIT
Signed-off-by: Oli Evans <oli@tableflip.io>

Merge pull request filecoin-project#455 from filecoin-project/post-in…

…put-types

use sensical types in GeneratePoSt and VerifyPoSt pseudocode