Document HFC

[dnadales]

• Follow-up on the previous point: Distill HWW section on HFC interface #375
• Haddocks for certain aspects of the HFC status quo #428
• docs: draft HardForkCombinatorRollbackMonotonicityProposal.md #346 adds a relatively high-level description of potential improvements/changes to the HFC behavior.
• Expand tutorials with HFC era transitions #374
• Specify cross-era ticking/forecasting for Cardano #418

Part of #420