Generate a list for each branch documentation.

[dhebbeker]

Now the documentation for each branch is generated in a path, which corresponds to the branch name.

All directories in the branch gh-pages which do not correspond to a branch are deleted on push to develop or main.

An entry page is generated which links to the paths for the documentation of all branches.

The documentation for some branches may be unavailable at first. Those branches need to merge with develop and push. Only then the documentation will be available.

Resolves #42.

[dhebbeker]

• [x] the branch list should be updated on push to any branch