Created saved replies docs

What does this MR do and why?

Added the base for saved replies docs.

Screenshots or screen recordings

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How to set up and validate locally

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

MR 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 MR.

• Related to Saved replies documentation (#393516 - closed)
• Related to Saved replies frontend work (#355845 - closed)