Docs: Update frontend development guidelines for etag caching

What does this MR do?

Resolves #345916 (closed)

This updates the development guidelines for etag caching by adding more info on the following:

• Updating the etag resource dynamically
• Adding a visibility check to pause polling when the browser tab is not active
• How to test that it works

Other changes are made to fix the linter complaints for this section of the document.

Related issues

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course
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If you are only adding documentation, do not add any of the following labels:

• ~"type::feature"
• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

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