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]

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