Added a derivation and useful repo links to oracle overview

[MaxResnick]

Removed a dead link and added some details throughout. Added links to Tickmath.sol and Oraclelibrary from the Uniswap repo since they are useful for computing TWAPS. Also added an in-depth mathematical derivation for sufficient observation cardinality for reliable TWAP oracles.

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[willhennessy]

Thank Max! Connor can you please review?

[willpote]

Unfortunately most of the math here doesn't render correctly in the browser. The analysis is also feels too in depth for our overview. Closing.