docs: arbitrum updates

[jcstein]

• add high level overview from @Ferret-san into the introduction page https://github.com/celestiaorg/nitro/blob/celestia-v2.3.1/docs/celestia/docs.md
• link to the blobstream x contract table, in the blobstream section of nitro docs
• add context about oracle image, blobstream, fraud proofs, from @Ferret-san description
• add compatibility table of celestia-node version to nitro version, before install steps
• remove nitrogen page
• add mocha guide into devnet guide, making it 1 guide by default that shows how to use light node
• other tasks listed in the PR docs: arbitrum updates #1490