chore: Focus CONTRIBUTING.md on code contributions.

[winder]

Summary

Update CONTRIBUTING.md to be more focused on code contributions to go-algorand. The catch-all nature of the previous version was useful for cross-documentation, but made it less useful for it's main purpose of guiding potential core contributors.

Some specific themes to this PR:

• Remove content about other repositories. It was out of date and a distraction for go-algorand contributors.
• More information about what should be done when preparing a PR.
• More information about what to expect during peer review, and an attempt to set expectations.

Test Plan

Not applicable.

[bbroder-algo]

much better

[winder]

@abebeos updated with some guidance about non-code contributions.

[algoanne]

typos/grammar. But other than that I like it all content-wise.

[gmalouf]

This is much more compact, focused, and clear.