Consider moving Proof Type section to Annex

[paulbastian]

Proof Types section has becoming very large and it makes the Credential Endpoint section hard to read. Consider moving it to Annex or separate chapter?

[Sakurann]

discussed in the WG call, @awoie supported. no strong opinions otherwise.

[jogu]

I had a look after the call and would definitely not object to moving that section, I agree in the current position it does somewhat make the credential endpoint section harder to read/understand.