This commit adds support to build individual `pdf` files for each source document (.md etc) using the generic sphinx `pdflatex` builder. It can be enabled for `builder=pdflatex` using the `cli` option `--individualpages`, for example:

```bash
jb build <project> --builder=pdflatex --individualpages
```

A use case is for lecture sets, where each file is an independent lecture,
and also to support downloadable PDFs for websites. 

Individual `pdf` files can be specified manually in `sphinx` configuration through the [latex_documents](https://www.sphinx-doc.org/en/3.x/usage/configuration.html#confval-latex_documents) tuple configuration. 
This commmit implements an auto-builder that collects the source files and titles and updates the `latex_documents` tuple when calling `sphinx_build`

An example for [myst-nb.example_project](https://github.com/executablebooks/myst-nb.example-project/pull/8/files

) shows how these tuples can be constructed. 

Co-authored-by: Chris Sewell <chrisj_sewell@hotmail.com>