`@remotion/webcodecs`: Document writers and consider interface

[JonnyBurger]

We document the reader interfaces in media-parser.

We should also document the writer interfaces:

• nodeWriter (in media-parser package)
• webFsWriter (in webcodecs package)
• bufferWriter (in webcodecs package)

[JonnyBurger]

Works, but TODO:

• Document webFsWriter, nodeWriter (in media-parser), and bufferWriter