Review and streamline contributor and developer documentation

[jeffpaul]

Description

Following on from the work in #42 and #53, let's review existing documentation files for clarity, conciseness, and proper organization. The goal is to simplify contributor onboarding, reduce duplication, and ensure each file serves a clear, distinct purpose.

Files to review

• CONTRIBUTING.md
• docs/DEVELOPER_GUIDE.md
• docs/DEVELOPER_GUIDE.md
• .github/SUPPORT.md

Goals

• Simplify and shorten each document while keeping essential details.
• Identify redundant or outdated content and remove or relocate it as appropriate.
• Ensure each file’s purpose is clear (for example, contribution workflow vs. local dev setup vs. support process).
• Add short cross-references in README.md so users can easily find the right document based on their need (contributing, developing, testing, or seeking support).

Acceptance criteria

• Each reviewed file is shorter, clearer, and focused on a single topic.
• Unnecessary or duplicate content is removed or consolidated.
• README.md includes direct links to these documents with short one-line descriptions.
• All links and file references verified to work correctly.