Document valid encoding values

[elshize]

All available encoding values should be documented. There is somewhat incomplete documentation here: https://pisa.readthedocs.io/en/latest/compress_index.html (only mentions actual values of the parameter for some compression techniques).

I think we should do the following:

• complete the documentation by adding missing values,
• make them stand out more by maybe documenting them right below each header,
• linking that documentation page in the CLI description of --encoding

Ideally, we could print all available values in the --help but some refactoring of how we define them in the code might be in order first. (like getting rid of the macro, try using templates instead) so I suggest the above at the moment.