Feat: [demo-feature-banners]: Add demo feature availability banners

[rtblack2701]

Description

This PR introduces a reusable featureflag Liquid block plugin that allows documentation writers to indicate feature availability (Enterprise, Cloud, or OSS) and associate version constraints. This improves consistency, reduces duplication, and centralizes formatting for feature-level callouts across the docs.

Authors can now use the following syntax:

{% featureflag availability="enterprise" version="3.5+" %}
Only available in Kong Gateway Enterprise {version} and later.
{% endfeatureflag %}

• The {version} token is automatically replaced with a clean version string (e.g. 3.5, stripping .x or +).
• An appropriate icon (🏢, ☁️, 🌱) and label are rendered in a styled block to increase visibility and maintain a consistent tone.
• Inline badge support was intentionally excluded to keep implementation and visual standards simple.

Why? This provides a scalable, low-maintenance way to mark feature gating across versions or product editions and supports future refactoring or automation efforts.
⸻

Preview

Testing instructions

To test:

1. Add the block to any Markdown file under app/_src/ using the above syntax.
2. Verify that the rendered output includes:

• Correct icon and label.
• Version displayed in parentheses.
• {version} properly replaced in content.
• No rendering issues or extra icons.

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