"Drivers" vs "Helpers" is not so helpful as a top-level distinction

I find that dividing libraries between device "drivers" and "helpers" is not so helpful. I often have to think about which top-level category to look in. I would rather just get rid of this distinction. For example, "Board-specific Helpers" could just be "Board Libraries".

Even the filename `drivers.rst` could be changed to `libraries.rst`.

Start with drivers.rst and inspect its top-level headings and any references to the filename. Search the repository for the Drivers and Helpers labels and drivers.rst; the work is done when the documentation uses the unified library naming consistently and links still resolve.

Written by the indexing model from the issue text.