Center and enlarge the PWMenu Wiki button

[newfpv]

What changed

• Replaced the inline Markdown badge with a GitHub-compatible centered HTML block.
• Increased the Wiki button height to 52 pixels.
• Strengthened contrast with a dark label background and cyan text/logo treatment.
• Kept the same Wiki destination and accessible alt text.
• Made no other README or code changes.

Why

The Wiki link is the primary call to action on the text-first project page. It should be immediately visible, visually stronger, and centered above the feature description.

Validation

• git diff --check
• Shields.io returned HTTP 200 with an SVG response for the new badge URL.
• Full regression suite: 59 tests passed.