doc: Add Makefile and repository to conf.py

[gastmaier]

Pull Request Description

Adds the default Sphinx Makefile with vars SOURCEDIR and BUILDDIR set. I need this file to exist because doctools needs to know the source and build paths. Set values are consistent with the ones at documentation.sh. Created it in the doc/sphinx/source path instead of doc/sphinx because it would be the only file at the latter path. Also, includes repository var to conf.py, also used by doctools.

PR Type

• [ ] Bug fix (change that fixes an issue)
• [X] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

• [X] I have followed the Coding style guidelines
• [X] I have performed a self-review of the changes
• [X] I have commented my code, at least hard-to-understand parts
• [ ] I have build all projects affected by the changes in this PR
• [ ] I have tested in hardware affected projects, at the relevant boards
• [X] I have signed off all commits from this PR
• [ ] I have updated the documentation (wiki pages, ReadMe etc), if applies