Added search instruction to Search Messages sidebar

[dhairyashiil]

Brief Title

Added search instruction to Search Messages sidebar

Acceptance Criteria fulfillment

• Instruction Display: When the view type is "Search Messages", a section with a markdown formatted message appears at the top of the sidebar.
• Styling: The instruction should have its own custom style handling dark and light themes.
• Rendering Condition: The instruction should only be rendered when the title of the view is Search Messages.

Fixes #909

Video/Screenshots

and

PR Test Details

Note: The PR will be ready for live testing at https://rocketchat.github.io/EmbeddedChat/pulls/pr-910 after approval. Contributors are requested to replace <pr_number> with the actual PR number.