Some doc fixups

[gadomski]

Description

• I moved the doc files up to the top level and linked to them from docs — this lets the links work from https://github.com/worldbank/blackmarblepy/blob/main/README.md as well as the docs website.
• Fixed a small typo in the README intro (extra ,)
• Removed the "no display" tag from the example notebook for the imports block — to me, it is very confusing to read a notebook and not know where a function came from
• Ignore docs/api since those are autogenerated

Motivation

• These arose while doing my pyOpenSci review: BlackMarblePy Submission  pyOpenSci/software-submission#207 (comment)

Type of change

• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings