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

<img
src="https://github.com/user-attachments/assets/d559710e-5e0d-44af-847f-7f9128b5f881">

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

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *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includes what's a link to the new unpublished blog post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)
> 
> <comments>
> </comments>
> 


</details>

Fixes #398

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Co-authored-by: madolson <34459052+madolson@users.noreply.github.com>