Embed mermaid code in markdown files

[shaun-nx]

Proposed changes

This change is a followup to #977 as part of nginx/nginx-gateway-fabric#3665 to ensure the diagrams are actually updated.
This change also updates basic-routing.md and advanced-routing.md to add mermaid diagram code inline in both markdown files.

Checklist

Before sharing this pull request, I completed the following checklist:

• I read the Contributing guidelines
• My branch adheres to the Git conventions
• My content changes adhere to the F5 NGINX Documentation style guide
• If my changes involve potentially sensitive information¹, I have assessed the possible impact
• I have waited to ensure my changes pass tests, and addressed any discovered issues

Footnotes

1. Potentially sensitive information includes personally identify information (PII), authentication credentials, and live URLs. Refer to the style guide for guidance about placeholder content. ↩

[github-actions]

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