Document docs.rs metadata on the 'about' page.

[frewsxcv]

Someone online asked me what this metadata section was in one of my Cargo.toml files, and it'd be helpful if this documentation was somewhere on the docs.rs site.

Docs/example taken from:

• https://github.com/onur/docs.rs/blob/8a31bf722cb416fec86866e3dd54ba59762e31fa/src/docbuilder/metadata.rs#L15-L27

[onur]

Thanks!