Codify style guide

[digama0]

To ease new contributions, we should write down the style rules (indentation, usage of trailing punctuation and structuring tactics) so that we have a place to point contributors to. Currently the most complete source of this information is the lean pretty printer, but this also has many bugs or issues that we were not able to fix directly in the pretty printer architecture, and mathlib maintains some scripts to address shortcomings in the pretty printer.