Add banner for Valkey 9.0 announcement (#399)

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includes what's a link to the new unpublished blog post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

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

✨ Let Copilot coding agent [set things up for
you](https://github.com/valkey-io/valkey-io.github.io/issues/new?title=✨+Set+up+Copilot+instructions&body=Configure%20instructions%20for%20this%20repository%20as%20documented%20in%20%5BBest%20practices%20for%20Copilot%20coding%20agent%20in%20your%20repository%5D%28https://gh.io/copilot-coding-agent-tips%29%2E%0A%0A%3COnboard%20this%20repo%3E&assignees=copilot)
— coding agent works faster and does higher quality work when set up for
your repo.

---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: madolson <[email protected]>

Author: Copilot

config.toml

@@ -19,6 +19,7 @@ highlight_code = true
 "valkey-conf" = "valkey.conf"
 
 [extra]
+banner = "**What's new in Valkey 9.0?** Discover new features and improvements. [Read the announcement](/blog/introducing-valkey-9/)."
 command_description_path = "../build-command-docs/"
 command_json_path = "../build-command-json/"
 command_bloom_json_path = "../build-bloom-command-json/"