Svelte docs: document `defaultValue`

[theetrain]

Special element attributes defaultValue and defaultChecked (maybe there are others?) don't appear to be documented. I could see this being its own page, or an addendum to some existing pages.

I see these as special since although they've available through the DOM API, Svelte allows them to be set as attributes that work well with bind:value or assigning value to either a string or nullish value.

Applicable pages:

• Basic markup: https://svelte.dev/docs/svelte/basic-markup
• New page, "Enhanced markup"?
• Progressive enhancement: https://svelte.dev/docs/kit/form-actions#Progressive-enhancement
• Tutorial, Bindings: https://svelte.dev/tutorial/svelte/text-inputs