Add banner for Valkey 9.0 announcement

[Copilot]

This PR adds a banner to the top of the Valkey website announcing the release of Valkey 9.0 and linking to the release blog post (PR #390).

Changes

• Added banner configuration to config.toml with a concise message about Valkey 9.0
• The banner links to the blog post at /blog/introducing-valkey-9/
• Uses the existing banner infrastructure (templates, styling, and JavaScript) already present in the website

Banner Features

• Message: "What's new in Valkey 9.0? Discover new features and improvements."
• Dismissible: Users can close the banner using the X button on the right
• Persistent: Banner dismissal is remembered in localStorage for 24 hours
• Responsive: Works across all pages and devices, displays on a single line without wrapping

Screenshot

The banner appears at the top of every page with white text on a purple background (#2D2471), consistent with the existing banner design system.

Implementation Note

This implementation leverages the complete banner system that was already built into the website templates. No template, styling, or JavaScript changes were needed - only the configuration value was added to enable the banner.

Fixes #398

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add a new banner for a what's new for Valkey 9.0</issue_title>
<issue_description>Add a banner to the top of the Valkey website that includes what's a link to the new unpublished blog post #390. </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #398

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[stockholmux]

lgtm