Document published editor extensions

[patrick91]

What changed

• mark the VS Code extension as published on the Visual Studio Marketplace
• make the Visual Studio Code installation link explicit
• document Cursor's Open VSX-compatible gallery and the temporary VSIX sideloading path

Why

The v0.1.5 release is live on Microsoft's Marketplace, but Cursor does not consume that registry. A clean Cursor CLI install from patrick91.django-lsp returns Extension not found; Cursor installation remains a VSIX sideload until Open VSX publication is available.

Validation

• installed django-lsp-0.1.5-darwin-arm64.vsix from the public GitHub release into isolated Cursor data and extension directories
• confirmed Cursor lists patrick91.django-lsp@0.1.5
• confirmed the bundled executable is ARM64 and reports django-lsp 0.1.5
• sent a real LSP completion request to the released binary and received author__team
• cargo test --test markdown_docs
• git diff --check