Only build toc files and add pytest_cache to exclude patterns

[finsberg]

When I try to build the book from the root directory, the default behaviour is to recursively traverse through all the subfolders an search for notebooks and markdown files. Since I usually create a virtual environment in this directory this is a problem because it will try to execute all notebook files inside the virtual environment. To change this behaviour I set

only_build_toc_files: true

in _config.yml (both in this repo and in the generated repo).

Moreover, even though this flag is set to through it still tries to build the files in my pytest_cache folder (which appears after pytest has been run), and this folder contains markdown files as will. Therefore I am also adding this folder to the exclude_patterns