Documentation restructure

[martyn-fewtrell]

PR Classification

Documentation restructure and content update.

PR Summary

This pull request restructures the documentation repository for improved clarity, navigation, and consistency, updating content and internal links to match a new organizational scheme.

• Adds new README files for each top-level area (communities, policies, standards, guidance, reference) with clear entry points and related links.
• Introduces a comprehensive documentation guidance file detailing documentation modes, structure, and contributor checklists.
• Updates the main README to serve as a content map with audience journeys and area links.
• Refactors and relocates policy, standard, and guidance documents, updating references and internal links throughout.
• Rewrites CONTRIBUTING.md to align with the new structure and document mapping guidance.

[snyk-io-eu]

✅ Snyk checks have passed. No issues have been found so far.

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✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

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[elerivaliant]

What I like about this:

• Relative links
• More logical structure

Less happy with:

• The introduction of a lot of pages that are just lists of content. It feels like you would never be able to edit anything here again without AI having to be involved to make sure you haven't broken links.