Add runnable Markdown BlockEditor example

#2 in Riparion/riparion-editor — merged 2026-06-02

What

Adds a complete, runnable browser demo of BlockEditor.

• examples/markdown.rs — a full Dioxus web app that plugs pulldown-cmark into render_block, seeds a document (headings, lists, fenced code, an [[embed]] line), and treats [[…]] lines as atomic blocks rendered as a card. Ships a small inline stylesheet so it's legible without a CSS framework.
• Cargo.toml — pulldown-cmark as a [dev-dependencies] (example-only; the crate stays renderer-agnostic) and an [[example]] entry gated on required-features = ["web"]. The gate keeps the default host build — and CI's cargo clippy --no-default-features --all-targets — skipping the example, since dioxus::launch needs a renderer feature to compile.
• README.md — a "Running the example" section.

Run it

dx serve --example markdown --features web

Verification

• cargo build --example markdown --features web --target wasm32-unknown-unknown ✅
• cargo clippy --example markdown --features web --target wasm32-unknown-unknown -- -D warnings ✅
• cargo clippy --no-default-features --all-targets — confirmed it does not build the example ✅
• cargo fmt --all -- --check ✅
• dx build --example markdown --features web --platform web — full wasm bundle built successfully ✅

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