Fix casing in API documentation

Fix typo

Document more resources about field mapping

Rename `KnownField::Website` to `KnownField::PerformerWebpage`

For consistency with `KnownField::PublisherWebpage`. This is also probably
what the Vorbis Comment field `WEBSITE` is used for (considering there's
the distinct field `WWWPUBLISHER` for the publisher webpage).

Remove grammatically incorrect sensence from documentation

Add FOURCC mapping for Versatile Video Coding

Fix casing of Ogg

It is always written as Ogg on https://xiph.org/ogg/doc/index.html (and not
OGG).

Fix duration and bitrate calculation of MP3 files via XING header

* Calculate the duration independently of the bitrate which is supposedly
  more accurate
* Fix conversion factor for computing bitrate
* Use real size if it differs from size specified in Xing header; this way
  the bitrate is consistent with MediaInfo and other tools (there is
  possibly still a bug in the way the size is read from the Xing header,
  though)

Use generic `toInt()` function which relies less on compiler optimiza…

…tions

Detect zstd compressed files