text: Title: add action to post binder link on each PR
  Body: we want to create a github action that posts a link to binder whenever someone opens a PR, this will allow us to easily test things
  
  some notes:
  
  1. since this requires a github token, I believe the trigger should be pull_request_target
  2. the host should be binder.ploomber.io
  3. we also need to add an environment.yml, similar to [this one](https://github.com/ploomber/jupysql/blob/master/environment.yml)
  4. and a [postBuild](https://github.com/ploomber/jupysql/blob/master/postBuild) like this
  
  more information
  
  https://mybinder.readthedocs.io/en/latest/
  https://mybinder.org/
  
  some related code:
  https://github.com/jupyterlab/maintainer-tools/blob/main/.github/actions/binder-link/action.yml