Add ai gateway observability guide

[Dimagidhp]

Purpose

The ai-gateway-observability sample has no companion guide. This adds one, covering how to observe both LLM and MCP traffic on the AI Gateway from the Insights dashboards in the AI Workspace.

Resolves wso2-enterprise/apim-gtm#724

Checklist

• Verified that llms.txt (located at en/docs/llms.txt) is updated for AI readiness content.
• Ensured meaningful alt text for images and that the information contained in an image also appears in text so the relevant info exists in the body of the document.
• Added or updated frontmatter for the respective pages.

Goals

Give readers one console-based guide that answers where their token budget went, which guardrail blocked a request, and which tools their agents called — for LLM and MCP proxies side by side.

Approach

New page at en/docs/guides/ai-and-mcp/observe-ai-gateway-traffic.md, added to mkdocs.yml nav in three locations.

Five steps, all in the AI Workspace console: send traffic, set the scope, read the Overview dashboard, read LLM APIs Analytics, read MCP Analytics. Five screenshots captured from a live gateway with real traffic. Links the sample as Try the sample.

Documentation

This PR is the documentation.

Samples

Companion sample: https://github.com/wso2/api-platform/tree/main/samples/ai-gateway-observability — standalone, runs the gateway against a mock model. The guide links it but does not mirror it.

Security checks

• Confirmed that this PR doesn't commit any keys, passwords, tokens, usernames, or other secrets? yes

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N/A for this PR: User stories, Release note, Training, Certification, Marketing, Automation tests, Related PRs, Migrations, Test environment, Learning — documentation-only change.

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