Document the EXIF, IPTC, XMP metadata handling

[fkaminski]

Extend the previous entries in: https://docs.contao.org/manual/en/file-manager/meta-data/

with features from https://github.com/contao/image/releases/tag/1.2.0 respectively regarding #5837 with information like:

• Feature overview
• Differences between 4.13.x & 5.2.x
• Configuration options etc