Feat: Add action to trigger build

[ianhi]

Problem

I would like to use a github actions artifact from a build step in the readthedocs build. Specifically building a wheel for a rust python project which takes a decent amount of time. These are already built in a github actions build step so it seems wasteful to build them again on readthedocs, and it takes extra time.

Potential Solution

Add an action that that makes webhook request in the same way as the github webhook integration. This action could then be a step in a workflow file that depends on a prior build step.

I don't think it's necessary for this action to incorporate management of the artifacts, the user can do that on their own in one of the build or install steps.

Alternative Considered

I tried getting this work with the generic webhook API but ran into issues trying to figure out how to point at a pull request branch. It's not obvious to me that it's possible. Further it would be nice to have the build still show up as one of the checks on a pr.