Set up a doc formatter

[agoscinski]

From discussion in PR #179: We have a doc linter, but no formatter for the doc rst files (like black for code) . But all existing formatter are not so perfect for our purpose and need a bit more tuning or/and some fixes.

docstrfmt:

1. does not recognize citation and citation-reference sphinx nodes (this is a bug of rstfmt)
2. it is quite aggressive (creates smaller case reference)

For 1. we need to open an issue at the respective repo. For 2. we need to figure how to config it