Add "Typing Guidance for Python Libraries" document

[srittau]

By Eric Traut, source: https://raw.githubusercontent.com/microsoft/pyright/main/docs/typed-libraries.md

See srittau/peps#94 for background.

This is a pristine copy of the original document. It needs to be
converted into RestructuredText and linked from the index as a next
step.

[srittau]

Cc @erictraut

[stefanv]

@srittau Another route would be to configure Sphinx to parse Markdown files, since people seem to enjoy writing in that format and we want to encourage contribution.

Option 1: Use MyST which supports both markdown and RST.

Option 2: Use recommonmark by adding this and this line.

[srittau]

While I usually prefer markdown, I'd prefer to keep using ReST exclusively here, for a few reasons:

• To stay consistent within the documentation.
• It's the standard for Python documentation.
• To allow easy reshuffling between documents.
• To be able to use advanced features like standardized crosslinks to Python documentation and index items.

[stefanv]

OK, here is the Pandoc conversion to RST, in case that's helpful to start with.

[srittau]

Thanks! That's quite useful indeed.

[srittau]

Bumpity bump. I'd like to start integrating the document into our docs structure. We can have all discussions about the contents then. For now, I'd like this original version to be in version control.