[WIP] Restructure and expand CONTRIBUTING

### Changelog:
* Restructure the `Fixing an issue` section:
  * Juxtapose sections about original and planned features.
  * Write a detailed guide on submitting bugs.
* Include code documentation and PR documentation in the requirements section.
* Install consistent markdown.
* Occasionally improve the prose.
* Limit the line length to 79 chars, save for long links.
This is reported speech ("... tell us what error you are observing"), as denoted by both the word order and the absence of the question mark.