Resolve "Diagram characters show in search"

What does this MR do and why?

WHAT

• This MR fixes the issue where search result snippets show mermaid chart code IF there's a query match in that code block.
• Adds data-elastic-exclude tag to mermaid codeblock render template.

WHY

• Search results snippets should be highlighting just plain content not code. This can be distracting and not really relevant/necessary.

Screenshots, screen recordings, or links to review app

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment.
2. Check out this branch.
3. Make sure search works as is, highlight snippets don't contain code, filters and pagination works as-is.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.

Closes #493 (closed)