Prepare GoodDollarStaking for audit

[sirpy]

• [x] modify methods totalRewardsPerShare, getUserPendingReward to return tuple with GOOD/G$ rewards data
• [x] plan tests for StakingRewardsFixedAPY - ie just natural langauge specs
• [x] implement the tests
• [x] copy unit tests from governancestaking and adapt to new function names, make sure they pass
• [ ] add docs and link to notion specs
• [x] plan new unit tests, go over code and write tests just in natural language
• [x] learn about hardhat "fixtures" and use that in your tests, to start each test from a specific known state
• [x] implement unit tests

[sirpy]

@Shloyem i've merged the first PR. create a new PR. (branch from gooddollar-v3) TODO:

• [x] finish StakingRewards tests. (i've added rewardsDonated to stakerinfo, make sure you test in the relevant tests)
  • [x] copy unit tests from governancestaking and adapt to new function names, make sure they pass
  • [x] add docs and link to notion specs
  • [ ] plan new unit tests that are relevant for GoodStaking, we already tests the calculations, so mostly its about principle and rewards being transferred correctly, the undorewards happening when relevant etc
• [ ] implement new GoodStaking tests
• [ ] check places where we do division for possible div by zero cases we might not be accounting for

[sirpy]

@Shloyem i've merged the PR. the first 2 checkboxes are done. continue with the rest on a new branch and PR forked from gooddollar-v3 connected to this issue