Move docs from book/ to docs/ (#8469)

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docs/src/transaction.md

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+# Anatomy of a Transaction
+
+This section documents the binary format of a transaction.
+
+## Transaction Format
+
+A transaction contains a [compact-array](#compact-array-format) of signatures,
+followed by a [message](#message-format).  Each item in the signatures array is
+a [digital signature](#signature-format) of the given message. The Solana
+runtime verifies that the number of signatures matches the number in the first
+8 bits of the [message header](#message-header-format). It also verifies that
+each signature was signed by the private key corresponding to the public key at
+the same index in the message's account addresses array.
+
+### Signature Format
+
+Each digital signature is in the ed25519 binary format and consumes 64 bytes.
+
+
+## Message Format
+
+A message contains a [header](#message-header-format), followed by a
+compact-array of [account addresses](#account-addresses-format), followed by a
+recent [blockhash](#blockhash-format), followed by a compact-array of
+[instructions](#instruction-format).
+
+### Message Header Format
+
+The message header contains three unsigned 8-bit values. The first value is the
+number of required signatures in the containing transaction. The second value
+is the number of those corresponding account addresses that are read-only.  The
+third value in the message header is the number of read-only account addresses
+not requiring signatures.
+
+### Account Addresses Format
+
+The addresses that require signatures appear at the beginning of the account
+address array, with addresses requesting write access first and read-only
+accounts following. The addresses that do not require signatures follow the
+addresses that do, again with read-write accounts first and read-only accounts
+following.
+
+
+### Blockhash Format
+
+A blockhash contains a 32-byte SHA-256 hash. It is used to indicate when a
+client last observed the ledger. Validators will reject transactions when the
+blockhash is too old.
+
+
+## Instruction Format
+
+An instruction contains a program ID index, followed by a compact-array of
+account address indexes, followed by a compact-array of opaque 8-bit data. The
+program ID index is used to identify an on-chain program that can interpret the
+opaque data.  The program ID index is an unsigned 8-bit index to an account
+address in the message's array of account addresses. The account address
+indexes are each an unsigned 8-bit index into that same array.
+
+
+## Compact-Array Format
+
+A compact-array is serialized as the array length, followed by each array item.
+The array length is a special multi-byte encoding called compact-u16.
+
+### Compact-u16 Format
+
+A compact-u16 is a multi-byte encoding of 16 bits. The first byte contains the
+lower 7 bits of the value in its lower 7 bits.  If the value is above 0x7f, the
+high bit is set and the next 7 bits of the value are placed into the lower 7
+bits of a second byte. If the value is above 0x3fff, the high bit is set and
+the remaining 2 bits of the value are placed into the lower 2 bits of a third
+byte.
+
+## Account Address Format
+
+An account address is 32-bytes of arbitrary data. When the address requires a
+digital signature, the runtime interprets it as the public key of an ed25519
+keypair.