docs: explain Celestia-specific GRPC APIs (#2175)

Motivation:
celestiaorg/celestia-app#5147 (comment)

Author: rootulp

README.md

@@ -4,6 +4,9 @@ celestia-core is a fork of [cometbft/cometbft](https://github.com/cometbft/comet
 
 1. Early adoption of the ABCI++ methods: `PrepareProposal` and `ProcessProposal` because they haven't yet landed in a CometBFT release.
 1. Modifications to how `DataHash` in the block header is determined. In CometBFT, `DataHash` is based on the transactions included in a block. In Celestia, block data (including transactions) are erasure coded into a data square to enable data availability sampling. In order for the header to contain a commitment to this data square, `DataHash` was modified to be the Merkle root of the row and column roots of the erasure coded data square. See [ADR 008](https://github.com/celestiaorg/celestia-core/blob/v0.34.x-celestia/docs/celestia-architecture/adr-008-updating-to-tendermint-v0.35.x.md?plain=1#L20) for the motivation or [celestia-app/pkg/da/data_availability_header.go](https://github.com/celestiaorg/celestia-app/blob/2f89956b22c4c3cfdec19b3b8601095af6f69804/pkg/da/data_availability_header.go) for the implementation. Note on the implementation: celestia-app computes the hash in prepare_proposal and returns it to CometBFT via [`blockData.Hash`](https://github.com/celestiaorg/celestia-app/blob/5bbdac2d3f46662a34b2111602b8f964d6e6fba5/app/prepare_proposal.go#L78) so a modification to celestia-core isn't strictly necessary but [comments](https://github.com/celestiaorg/celestia-core/blob/2ec23f804691afc196d0104616e6c880d4c1ca41/types/block.go#L1041-L1042) were added.
+1. The GRPC server exposed by CometBFT was modified to expose Celestia-specific APIs:
+    - `BlockAPI`
+    - `BlobstreamAPI`
 
 
 See [./docs/celestia-architecture](./docs/celestia-architecture/) for architecture decision records (ADRs) on Celestia modifications.