Proposal: restructure docs using diataxis framework

[mefellows]

Background

The Pact documentation is maintained by a diverse set of Pact project maintainers, including many external contributions (which we are very grateful for). However, it suffers from issues of consistency (writing, diagrams), coherence (some sections are in non-obvious places), and lack of a structured plan.

Documentation should be awesome, and we're not quite meeting that bar.

Proposal

1. Restructure the docs using https://diataxis.fr/ as a guide
   1. Identify and document critical gaps
2. Update all diagrams to mermaid.js (code first, repeatable)
3. Create a lightweight style guide to improve writing consistency
4. Review links to smartbear commercial assets, ensuring clarity and transparency of the differences between OSS and commercial assets.
5. [optional] Introduce a review step that uses AI (e.g. opencode + ollama) to ensure documentation changes adhere to the style guide