Improve fungible docs (proposed template design)

[andrew-fleming]

This PR proposes to improve the fungible token docs continuing from this discussion. It doesn't change the ToC on the right side bar as originally proposed, however

Proposed changes:

• Add Ledger and Witnesses sections
• Separate circuits by export vs internal
• Move internal _update to bottom of module so it matches API

LMK what you guys think and/or if you have other ideas @0xisk @emnul

Fixes #180

[emnul]

So in general we want the order of the API section to match the ordering of the Compact circuits?

[andrew-fleming]

So in general we want the order of the API section to match the ordering of the Compact circuits?

I think so unless we have a stronger reason to do it another way

[emnul]

I think we should also have a dependency section to make sure users know to include Initializable and Utils in their project directory . Otherwise the contracts will fail to compile.