chore: docs: add user contract api docs

[jennijuju]

This work is heavily based of @longfeiWan9's work at https://github.com/longfeiWan9/BuiltIn-Actor-API

Todos before merge:

• Miner WithdrawBalance missing method num
• add datacap mint method
• add datacap destroy method
• update datacap Granularity docs (see update Granularity export name  #1197)
• audit the method numbers in README and the docs
• add hyperlinks to each method in the mega table

[longfeiWan9]

@jennijuju I have no access to directly push to your branch in this repo. Will you grant me access?

[jennijuju]

@jennijuju I have no access to directly push to your branch in this repo. Will you grant me access?

Done

[longfeiWan9]

Update this PR based on the checklist.
And then find out that we do not have ethaccount and evm actor docs, I already added ethaccount.md. But I have not write docs for evm actor since I still have not figure out what is it and how will it be used.

[jennijuju]

Update this PR based on the checklist.
And then find out that we do not have ethaccount and evm actor docs, I already added ethaccount.md. But I have not write docs for evm actor since I still have not figure out what is it and how will it be used.

for evm actor it doesn't have exported method tho!

[longfeiWan9]

@jennijuju added read-only flags according to the nature of built-in actor methods as well as read-only flags set in filecoin.solidity.

I think this is ready, maybe someone can review and sign off to merge ?

[anorth]

Rubber stamp. I haven't reviewed all the docs for accuracy. In this case we're deciding that something is better than nothing.

[ZenGround0]

@jennijuju we want this merged into master right?