schema hash code and docs are confusing #128

In iden3 docs it says the schema hash is created from the hash of the LD document bytes
https://docs.iden3.io/protocol/claim-schema/#schema-hash

but looking at the code linked from the docs here: https://go.dev/play/p/3id7HAhf-Wi
it only uses the schema @id to create the hash
lastly iden3 bytes to bigint reverses the byte order which is also confusing, when trying to recreate the hash using other code
This happens also when verifying the challenge on chain

1. iden3/polygon docs should be updated to reflect the correct way to calculate the hash
2. the go code should just ask for credential type+id instead of pasting the whole credential
3. the reversal of the bytes should be mentioned in multiple places, that would help future dev avoid this confusion