Motivation

Adds the option to define a GitHub label per project.

How to test

1. Clone the PR and build the docs.

2. Open an issue from the docs. The "documentation" label should be assigned to the issue.

3. Add the following option html_theme_options in docs/source/conf.py:

   html_theme_options = {
   (...)
   "github_label": "bug",
   }
   

4. Rebuild the docs.

5. Open an issue form the docs. The bug label should be assigned to the issue.