Re-architect Guidelines using Astro

[kfranqueiro]

This splits the guidelines into individual files for the following:

• Acknowledgements
• Groups
• Guidelines
• Requirements/assertions
• Glossary terms

These are then reassembled and built into guidelines/index.html by Astro.

Markdown is used for prose, and extensions are implemented to make common tasks more convenient (e.g. decision trees, editor's notes, definition links).

• Preview
• HTML Diff

Included in this PR

• Top-level guidelines folder containing groups, guidelines, requirements/assertions, terms, and acknowledgements, each split to separate files
• README documentation on commands, directory structure, data formats, and Markdown extensions
• Updated github action, which:
  • Runs the new build for guidelines, explainer, and requirements
  • Subsequently runs these through spec-generator
  • Reuses the existing build to produce the howto directory in the output folder
  • Note: The action is now structured such that GitHub Pages should be configured to use GitHub Actions as its source, rather than a branch (so it doesn't need to consume extra space when cloning the repository)

This PR also already incorporates the changes of the following open PRs:

• Fix typos in guidelines/index.html #274
• Update contributors #275
• Fix grammatical issues in guidelines/index.html #276

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