Restructure docs/site to Diataxis and cover v0.7.0 API

[kommundsen]

Description

Reorganizes docs/site/articles/ into a strict Diataxis four-quadrant layout and adds documentation for all v0.7.0 unshipped API surface.

Structure changes:

• Replaces the flat guides/ bucket with three purpose-built directories: how-to/, reference/, explanation/
• Removes installation.md, getting-started.md, troubleshooting.md, configuration.md — content redistributed into the correct quadrants
• All toc.yml files rewritten to match the new layout

New content (v0.7.0 coverage):

• tutorials/07-data-generation.md — DataGen.Sample/SampleOne/Stream walkthrough
• how-to/reproduce-a-failure.md, export-repros.md, use-data-gen.md, use-cli-tool.md
• reference/strategies.md — complete Generate.* catalog
• reference/string-strategies.md — Identifiers, NumericStrings, VersionStrings
• reference/extension-properties.md — .Positive, .NonEmpty, | operator
• reference/attributes.md — complete attributes reference (was missing)
• reference/formatters.md — JsonOutputFormatter, JsonLinesOutputFormatter
• reference/settings.md — merged settings reference including ExportReproOnFailure, ReproOutputPath
• explanation/shrinking.md, targeted-testing.md, example-database.md, property-based-testing.md

Also adds the diataxis skill to .claude/skills/diataxis/ for future documentation work.

Type of change

• Documentation / chore

Checklist

• dotnet test src/ passes
• New behavior is covered by tests (TDD: Red → Green → Refactor)
• Follows .editorconfig code style