Add Documentation for the Session Class

[JosueNina]

Add the documentation for the new Session class introduced in PR #8922

The Session class provides unified access to daily session data (OHLCV + OpenInterest) for any security type (Equity, Future, Option, etc.) through the property:
var session = Security.Session;
It is worth noting that the fixed window size was later changed from 2 to 3 to retain an extra day of historical session data.

[Martin-Molinero]

Update: session is disabled by default due to performance (ref QuantConnect/Lean#9046), can be enabled with security.session.size = X where X > 0