docs: rules must be wrapped in a workflow that compiles and executes them

[asulwer]

Reframe the README and docs so a Rule is presented as authored on its own
but never compiled or executed directly. A Workflow is the container that
owns the compiler and drives compilation and execution of its rules.

• README: restructure Quick start (define model, author rules, wrap in a
  workflow, compile and execute the workflow) and convert the
  multi-parameter example to a workflow.
• getting-started: rename steps to author/wrap, fix the stray standalone
  rule.Compile in the multi-parameter section.
• when-to-use-what: drop the 'use Rule, not Workflow' guidance and the
  standalone Individual Rule section; even a single rule runs in a workflow.
• rule.md: add a note that Compile/Execute are the per-rule machinery the
  workflow invokes; keep the method signatures accurate.
• architecture/aot-compatibility/compiled-delegate/security: present
  Workflow.Compile/Execute as the public entry points.
• Convert remaining standalone rule.Compile/rule.Execute snippets across
  example and API-reference docs to wrap the rule in a workflow.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01UAa2aqbD2kKWc71AdHywKC