docs: add examples and tutorials

[darioAnongba]

Summary

Create standalone examples showing how to use bdk-wasm in different environments.

Tasks

• Browser example (vanilla JS + Vite or Webpack)
• Node.js example (create wallet, sync, build and send tx)
• React/Next.js integration example
• Add examples/ directory with README
• Link examples from main README.md

[darioAnongba]

Taking this one. I am putting together an examples/ directory, environment-specific quickstarts, and README links, then I will open a draft PR once the docs are in shape.