docs(agentgateway): Add AgentgatewayPolicy long-form docs+accessLog/backend TLS /transformation examples

[GauravSRC]

Closes: #471

Summary

This PR adds a long-form documentation page for the AgentgatewayPolicy CRD and includes three example manifests demonstrating:

• Access logging to OpenTelemetry (OTLP) (examples/access-logging-agentgatewaypolicy.yaml)
• Backend TLS + TCP keepalive configuration using a Kubernetes Secret reference (examples/backend-tls-tcp-agentgatewaypolicy.yaml)
• Request/response transformation and prompt-guard response masking (examples/transformation-agentgatewaypolicy.yaml)

What I changed

• Added content/docs/agentgateway/agentgateway-policy.md -- long-form guide, schema excerpt, conceptual notes, and three example sections.
• Added example manifests in content/docs/agentgateway/examples/:
  • access-logging-agentgatewaypolicy.yaml
  • backend-tls-tcp-agentgatewaypolicy.yaml (uses secretRef; includes security note)
  • transformation-agentgatewaypolicy.yaml
• Added short safety notes in each example: “This is an example -- replace name/namespace/cert values for your cluster.”
• Ensured frontmatter includes title, toc: true, publishDate, and author.
• Verified local build: hugo server -D runs and hugo --minify completes locally.
• All commits include DCO signoff (Signed-off-by: Gaurav<gauravsingh70800@gmail.com>).

Why

AgentgatewayPolicy replaces the older TrafficPolicy and adds first-class support for access logs, backend configuration (TLS/TCP), and transformations. These docs provide copy/paste examples and security guidance to help users adopt the new policy.

How I validated

• Built the site locally (Hugo + Go installed). The page renders at: /docs/agentgateway/agentgateway-policy/.
• Confirmed no private keys or real certificates in repo (TLS example references a Kubernetes Secret).
• Confirmed commit DCO signoff.

Notes for reviewers

• These are docs + example manifests only; no code changes to the agentgateway runtime.
• Examples use placeholders and explicitly instruct users to replace name/namespace/secret values for their clusters.
• If you prefer the examples in a different path (e.g., static/examples/ or a new examples/ top-level folder), I can move them in a followup commit.

Testing locally

1. Clone my branch and run:
2. 2. Optional: hugo --minify to verify a production-like build.

Thank you --happy to iterate on wording, namespace choices, or to add a tiny CI workflow that runs hugo --minify if maintainers want a build check in the PR.