Improve contributor experience

[jnv]

• Add contrib guidelines
  • Mention some priorities and recommendations (e.g. follow Diataxis, refer to glossary)
• Add issue forms with relevant issues
  • E.g. bug with documentation, missing content etc.
• Collect feedback on the docs website?
• E.g. TypeScript, React has just thumbs up / thumbs down

[martyndavies]

Still relevant, picking up.