Streamline developer and contributing docs

[jeffpaul]

README.md currently includes sections for Developer Experience and How to Get Involved that overlap with each other and content in CONTRIBUTING.md. To reduce duplication and simplify the README, consolidate this guidance into the Contributing Guide and streamline the README to focus on project overview and usage.

As noted by @justlevine in #219 (comment), let's look to:

• Move detailed developer setup, workflows, and contribution guidance from README.md into CONTRIBUTING.md or DEVELOPER_GUIDE.md (whichever makes the most sense)
• Simplify the README by replacing those sections with short references that link to the Contributing or Developer guides
• Ensure CONTRIBUTING.md becomes the single source of truth for contribution workflows.
• Ensure DEVELOPER_GUIDE.md becomes the single source of truth for developer experience.
• Review and remove redundant or outdated content during consolidation.