docs(dom): add API usage examples - #1013

Contributes to #851.

What changed

• Adds a first-use @cycle/dom API example showing makeDOMDriver(), a DOM sink, and event selection with select().events().
• Documents that the driver accepts either a selector or an Element render target.
• Notes the document.body top-level VNode requirement for body-mounted apps.
• Keeps the checked-in rendered docs/api/dom.html in sync with docs/content/api/dom.md.

Verification

• node --check .scripts/make-api-docs.js
• git diff --check

Note

I attempted to regenerate the DOM API page with node .scripts/make-api-docs.js dom, but the existing docs toolchain fails before writing output because the installed dox dependency throws TypeError: parse is not a function. I therefore mirrored the Markdown addition into the rendered HTML without changing the existing generated API block.