Fix references to ItemValue, Picture, and PictureType in ApeTag

[Lepidopteran]

I was looking around in the ApeTag documentation and found the following to be pointing to invalid locations:

• ItemValue
• ItemValue::Binary
• Picture::from_ape_bytes
• PictureType::Undefined

I updated them to point to the right locations and tested it on my machine.
Feel free to let me know if any changes need to be made. :)

NOTE: This is a duplicate of a previous pull request I made, but with a verified the commit to make GitHub happy.

[Serial-ATA]

Thanks! I thought this would've been caught by broken_intra_doc_links, but I guess it doesn't work when the paths are specified like this. 🤔

I imagine more broken links exist, since a bunch of things were moved around in 0.19.0.

This is a duplicate of a previous pull request I made, but with a verified the commit to make GitHub happy

Don't worry about it, commits don't need to be signed.