Organize and clarify contribution content

[stumblefiend]

Improve clarity of doc contribution guidelines.

Improves content structure and clarity and moves the tone in a more formal direction:

• 16% improvement in Flesch-Kincaid readability scores
• 48% fewer words
• Shifts the tone more neutral by 50%

[stumblefiend]

@mihaimaruseac that's fine. Thanks again for the help here! One benefit of this change is that the need for a CLA becomes much more prominent. I thought this might be helpful since I saw that almost 10% of the pull requests since 2018 were labeled "cla: no".

By the way, I want to contribute in more meaningful ways, so let me know if there are some priority areas that need documentation help or if I should just stick to what appears in the forums under the "docs" tag.