Add spec doc explaining token limits

[robert-zaremba]

Summary

Create x/leverage/spec/borrow-rules.md document (feel free to propose other name if you find better one) explaining limits implemeted in recent PRs:

• feat: implement MaxSupply #1319 MaxSupply
• feat: improve close factor #1322 Improve CloseFactor
• feat: implement MaxCollateralShare #1329 MaxCollateralShare
• feat: implement MaxSupplyUtilization #1330 MaxSupplyUtilization
• feat: implement MinCollateralLiquidity #1331 MinCollateralLiquidity
• ... anything else missing

Try to link to proto files when possible, rather than copy pasting.

Let's try to do it after tagging v3.0.0-rc1.

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[toteki]

We can also add any commentary here on MaxSupply if we expect it will be replaced by something like MaxSupplyShare in the future. In that way we can keep those notes out of the code / proto.