Add API spec and update to 1.0

[h3xxit]

Summary by cubic

Publish the UTCP 1.0 API specification and reorganize documentation into a versioned, plugin-based API reference. This clarifies the compliance contract for UTCP clients and simplifies navigation.

• New Features

  • Added UTCP 1.0 API spec with a full reference (39 modules, 189 items) under docs/api and versioned_docs/version-1.0.
  • Documented core models, interfaces, and implementations (client, repository, search, variable substitution, exceptions).
  • Added plugin docs for communication protocols: HTTP (incl. streaming, SSE), CLI, MCP, and Text, plus an OpenAPI converter.
  • Introduced discovery and plugin loader docs, and variable loader support (.env).

• Refactors

  • Replaced legacy guides (providers and tool-caller docs) with the API reference; updated the Introduction to UTCP 1.0.
  • Tweaked security and UTCP vs MCP pages for accuracy.
  • Minor site style update (details background color).

[cubic-dev-ai]

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