Document `CompressedHexBytes` format in types.md and formats.md

The type `CompressedHexBytes` does not appear to be documented anywhere.

More generally, between formats.md, types.md, and the vectorgen docs, we do not have a single and maintained source of truth for these formats. This can be annoying downstream users.

Note that I am very happy with the change to CompressedHexBytes for ciphertext, and it is perfectly correct that my own tests will fail if they don't know how to handle a new format, but it would be nice for the documentation to keep up with these changes.

Read types.md, formats.md, and the vectorgen documentation to compare how formats are currently described. Trace the existing references to CompressedHexBytes and identify the maintained wording for its ciphertext format. Done means CompressedHexBytes is documented consistently across the relevant documents and the formats have a clear source of truth.

Written by the indexing model from the issue text.