DOCS: Checkpointing

[aakoshh]

Created a sequence diagram to showcase checkpointing, as promised in #286 (comment)

[adlrocha]

This is amazing, @aakoshh. It was long overdue. Thanks!

• The role of the IPC agent has been removed. It is now a library that Fendermint and the relayer can leverage to directly call the relevant child/parent subnets. It may be better to just remove it from the diagram and depict the relayer and Fendermint directly calling the relevant nodes?

• Also, it may be also worth having a diagram of how top-down messages are executed (@cryptoAtwill or me can pick this one in a follow-up PR).

[aakoshh]

The role of the IPC agent has been removed. It is now a library that Fendermint and the relayer can leverage to directly call the relevant child/parent subnets.

Okay, I only went by what happened in the now deprecated PRs where IIRC the agent had its own URL, but I could be wrong on that one. Let's fix the diagram when @cryptoAtwill again introduces the syncer. Until then we can treat this diagram as speculative.