document differences from the Cosmos SDK stack

[dynst]

Is your feature request related to a problem? Please describe.

This is somewhat surprising: #4627 (comment)

we don't have a flow to change consensus parameters anyway

Some parameters that are dynamically changeable with a vote on other Cosmos chains are only changeable with a hard fork on Penumbra. Documentation of that and other deliberate differences from the Cosmos SDK (e.g. there's no 'Community Tax' parameter to stockpile the native token in the Community Pool), and differences that are just stuff that can be implemented post-mainnet and aren't essential in the short-term, would be great.