Add github actions for automatic merge to gh-pages branch

[bhufmann]

Having such an action will automatically update the gh-pages branch after each update in master branch. This will make sure that the formatted trace server protocol page on https://theia-ide.github.io/trace-server-protocol/ is in sync with master branch.

[bhufmann]

This is resolved because the gh-pages webpage is built from the API.yaml file from the master branch using the link below in the index.html file.

https://raw.githubusercontent.com/theia-ide/trace-server-protocol/master/API.yaml

[marco-miller]

Thanks @bhufmann. I don't have enough permissions it seems to close this issue (still open).