This PR addresses https://github.com/a11yproject/a11yproject.com/issues/1319. It reorganizes the Contributing Guidelines page to:
- Lead with what content we do support, while moving what we no longer support lower down on the page.
- Consolidate our evaluation and rejection policies.
- De-emphasize GitHub-related information in anticipation of https://github.com/a11yproject/a11yproject.com/issues/1359.