Improve extension discoverability and overview

[SagaciousB]

Why

The display name was just "Buf" and the description was generic ("Visual
Studio Code support for Buf"), so the listing undersold what the extension
does and was hard to find by the terms people actually use - Protobuf,
.proto, and Protovalidate.

What

• Display name, description, and keywords now cover Protobuf, .proto, and
  Protovalidate/CEL - all capabilities the Buf language server provides today
  (Protovalidate/CEL hovers, completions, and go-to-definition shipped in buf
  v1.67.0+).
• Rewrote the README intro to lead with the language-server capabilities, and
  added a Protovalidate/CEL feature entry.
• Added Marketplace and Open VSX install links up top; sorted the reference
  links.

Metadata and docs only - no behavior change.

[CLAassistant]

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