Add README overview and full CEL/Go Template reference docs

[Copilot]

The repository lacked documentation beyond a 3-line README, making it difficult to understand capabilities or use the library effectively.

Changes

README.md

• Feature overview covering Go Template, CEL, and JavaScript expression engines
• Quick-reference tables with inline examples for both CEL and Go Template
• Library installation and usage snippets for all three modes (template, expression, struct walk)

CEL.md (new)

Full CEL reference including:

• Standard operators, type conversions, optional types (obj.?field.orValue("default"))
• Collection macros: filter, map, fold, all, exists, exists_one
• matchLabel / matchQuery helpers with pattern syntax (*, !, !*)
• Kubernetes (k8s.*), AWS (aws.*), math (including bitwise), random, regexp, strings, dates, filepath, JSON/JMESPath/JSONPath/jq

GO_TEMPLATE.md (new)

Full Go Template reference including:

• Delimiter customization, YAML multiline caveats, Helm escaping
• All function namespaces: base64, coll, conv, crypto, data, filepath, math, path, random, regexp, strings, test, time
• Pipeline examples throughout

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