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%

[mihaimaruseac]

Please don't use "add file"/"update file"/"fix file"/etc. commit messages. These are hard to reason about when looking at the history of the file/repository. Instead, please write explanatory git commit messages.

Also, I think this has to stay in the current format, it's a standard file used in most repos.

The commit message is also the title of the PR if the PR has only one commit. It is thus twice important to have commit messages that are relevant, as PRs would be easier to understand and easier to analyze in search results.

For how to write good quality git commit messages, please consult https://cbea.ms/git-commit/

[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.