Document CometMock/CometBFT differences/deficiencies

[p-offtermatt]

There should be documentation for the deficiencies where CometMock does not replicate the behaviour of CometBFT (yet).
For example, a folder with Markdown documentation.

[p-offtermatt]

v0.38:

• The POL Round is hardcoded to 0 when creating a new proposal TODO: link
• sendInternalMessage is not called when making a proposal
• The timeout in the context when subscribing to a websocket is fixed manually
• closeIfSlow is manually set to false
• We do not submit actual evidence - we just submit bare minimum evidence to tell the application someone misbehaved, but double-checking the actual evidence might not work
• For RequestPrepareProposal, use maxDataBytes = -1 (unlimited) since we have no mempool anyways