Write ADR for high-level pre-devnet architecture

[liamsi]

Summary

Describe the high-level architecture of the Celestia node and its different components (and then start the implementation accordingly).

Details

This could be several ADRs:

• high-level architecture describing the interaction with a tendermint node and the components

And more granular descriptions of:

• using API between Tendermint node
• peering with other Celestia nodes (incl discovery)
• interaction between Celestia nodes on a networking-level (DAS serving and the actual sampling)
  • serve sampling request
  • DAS itself
  • gossiping: headers, (fraud proofs)
  • serve Headers for syncing purposes
• proxying tendermint's RPC
• if fraud proofs, then ABCI component; else proxy state queries to Tendermint node
• storage

@renaynay @Wondertan feel free to amend / edit.