Fixing atomated generation of documentation.

[leojklarner]

Reference Issues/PRs

N/A

What does this implement/fix? Explain your changes

Simple PR that fixes various documentation-related issues and ensures that the documentation can be rendered on GitHub Pages.

What testing did you do to verify the changes in this PR?

Extensive local testing and bug-fixing of the Docs were performed. These changes are entirely cosmetic and do not affect any functional aspects of the codebase.

Pull Request Checklist

• [x] Added a note about the modification or contribution to the ./CHANGELOG.md file (if applicable)
• [x] Added appropriate unit test functions in the ./gauche/tests/* directories (if applicable)
• [x] Modify documentation in the corresponding Jupyter Notebook under ./notebooks/ (if applicable)
• [x] Ran python -m py.test tests/ and make sure that all unit tests pass (for small modifications, it might be sufficient to only run the specific test file, e.g., python -m py.test tests/kernels/test_graph_kernels.py)
• [x] Checked for style issues by running black . and isort .

[leojklarner]

As these changes are minor and just relate to the rendering of docs I will directly merge the PR.