docs: Document :bind attribute in html-syntax.md

[tormnator]

The :bind attribute is a useful feature for spreading object properties directly onto child components, but it has no dedicated documentation section in html-syntax.md.

What :bind does

Instead of passing individual named props, :bind spreads an object's keys directly as flat properties on the receiving component — so the child can reference them by name without any wrapper object.

Suggested addition

Add a new Binding objects section in docs/html-syntax.md, inserted right after the Conditionals section, covering:

• Basic usage: <component :bind="obj"/> spreads all keys of obj as props
• Shorthand for looping: <product-card :each="item in products" :bind="item"/>
• Using { this } to spread the current component's own data
• Reactivity: when the bound object is replaced, the child re-renders

Real-world example (from blog.html)

<author :bind="authors[author || authors.default]"/>

<author class="author row">
  <img src="{ img }">
  <h3>{ name }</h3>
  <a href="//x.com/{ username }">@{ username }</a>
</author>

The authors[...] object is spread, so img, name, and username are accessible flat inside <author>.

References

• Implementation: packages/nuedom/src/dom/node.js (getAttrData function)
• Tests: packages/nuedom/test/component.test.js (:bind and :bind this)