Modules documentation

[fkrause98]

We should improve the documentation from the modules we expose, which are:

• zks_wallet.
• zks_utils. (EDIT Moved to Document zks_utils module #119)
• zks_provider. (moved to Document zks_provider module #120)
• main_contract. (moved to Document main_contract module #121)
• eip712. (moved to Document eip712 module #122)

Useful links:

• The Rustdoc book.
• Cargo doc reference

[fkrause98]

Related #104

[fkrause98]

Working on zks_wallet module on #114