Add MkDocs build command to GitHub Actions

[davramov]

In PR #52 I added documentation created in MkDocs, but now I want to add a GitHub Action to build/deploy the documentation automatically whenever a new PR is merged. Once this is done, the documentation will then be hosted in a branch called gh-pages and accessible at https://als-computing.github.io/splash_flows_globus/

[davramov]

Documentation can now be found here: https://als-computing.github.io/splash_flows_globus/