Best Practices and Reference Implementations for API and AI Policies

[TristonianJones]

CEL was initially developed in response to needs for portable API policies,
but with the recent advent in AI standards like A2A and MCP, there's an
opportunity for CEL to expand its documentation to include best practices
and reference policies:

a) Formally document existing API policy practices and attributes
b) Propose new attributes and reference policies for AI use cases

CEL will expand the repo to include the specification as well as domain-specific
guidance and references to help teams get started with using CEL to
address their policy needs wherever CEL runs including: Java, C++, Go, Python,
and the community stacks for Rust and EcmaScript.