docs: improve project documentation

[leoafarias]

Summary

• correct misleading validation, JSON serialization, JSON Schema, API response, and date-codec examples
• expand configuration guidance and clarify the quickstart and API reference flow
• publish the adapter-authoring guides as navigable MDX pages
• add code of conduct, security, and support documentation
• add source-integrity coverage so tested snippets cannot silently diverge from the documentation

Why

Several examples had drifted from Ack's strict runtime semantics, including unanchored full-value patterns, integer inputs shown for double schemas, and a UTC value shown for the local-date codec. The JSON serialization page covered decoding but not the outbound encoding path, and two adapter guides were not part of the rendered documentation navigation. Existing documentation tests also reproduced corrected snippets without checking the literal source, allowing those differences to go unnoticed.

These updates make the documentation safer to copy, easier to navigate, and more complete for users and contributors.

Validation

• npx @docs.page/cli check
• dart test test/documentation --reporter=expanded — 137 tests passed
• dart run melos run analyze --no-select — all 6 packages passed
• browser verification across all 17 documentation routes at 1440×900 and 390×844
• Dart formatting and git diff --check

[docs-page]

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docs.page/btwld/ack~128

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