[misc] Submitting a proposal updates

[whichking]

Reorganization of the proposals section of the contribution guide, since the process is pretty different now. I've changed "Submitting a proposal" to cover only the process of submitting a proposal. All philosophy and cautions have been moved to "Proposing features and enhancements".

[sailsbot]

Thanks for submitting this pull request, @MadisonHicks! We'll look at it ASAP.

In the mean time, here are some ways you can help speed things along:

• discuss this pull request with other contributors and get their feedback. (Reactions and comments can help us make better decisions, anticipate compatibility problems, and prevent bugs.)
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