Create initial page for webp documentation.

I strongly based this on https://github.com/webmproject/libwebp/blob/36a6eea3bc2871c7f0166193a6fa42241af4a464/man/cwebp.1

@lsegovia do you know what the leftover items mean? Are my descriptions correct?

I also get the vague idea it might be good to reorder the export options, given the docs themselves say that some of them only work for lossy compression.