Add section on blocks.

Also includes some stubs for other sections
referenced as part of the block processing
section.

Author: Nicholas Clarke

src/ledger/concepts/blocks.md

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+# Ledger: Blocks
+
+Blocks are a fundamental component of all blockchains and represent one of two
+basic units of exchange between nodes (the other being [transactions](./transactions.md)).
+
+A block can itself be broken down into multiple parts:
+
+```mermaid
+
+block-beta
+block
+  columns 1
+  H["Header"]
+  block
+    columns 2
+    BB["Transactions"]
+    BW["Witnesses"]
+    BA["Auxiliary Data"]
+    BV["Transaction Validity"]
+  end
+end
+
+```
+
+See also the [Block CDDL](https://github.com/IntersectMBO/cardano-ledger/blob/master/eras/conway/impl/cddl-files/conway.cddl#L8)
+for the Conway era (the latest era at the time of writing).
+
+## The header/body split
+
+The most important distinction in the above diagram is that there is a split
+between the header of the block and the body. The general guiding principle
+behind this is the following:
+
+- The *header* contains the parts of the block relevant to the consensus
+  protocol.
+- The *body* contains the parts of the block relevant to the ledger.
+
+The full details and implications of this split are covered in
+[Implications of the Header/Body Split](../constraints/header-body-split.md).
+For now, we can assume that the contents of the header are not important for
+the ledger processing.
+
+The block body itself is split into four parts. This division is not conceptually
+necessary but is helpful for efficient processing:
+
+1. The 'Transactions' section contains the bodies of all transactions. This is
+    the only part of the block body necessary to compute the resulting ledger
+    state (see [The ledger state transition](../state-transition.md)) - that is,
+    provided that the block is _valid_ with regard to some _ledger state_, the
+    resulting new state can be computed using only the data in this section.
+
+2. The 'Witnesses' section contains the cryptographic witnessing necessary to
+    evaluate the _validity_ of the transactions contained in the block. For
+    more details on _validity_, please see the [Validity](../state-transition/validity.md)
+    section.
+
+3. The 'AuxData' section contains "auxiliary data" which is not processed
+    as part of the ledger state. It instead contains data which may be of use
+    to either users directly or other software interfacing with the chain. In
+    Shelley, this was limited to "metadata', which were indeed simply binary
+    blobs. In the Mary and Alonzo eras slightly more structure was provided to
+    allow including native and Plutus scripts respectively.
+
+4. The transaction validity contains a list of transactions in this block which
+    are [phase-2 invalid](../state-transition/validity.md). This is held
+    separately since this map is provided, effectively, by the node which
+    created the block, rather than by the creators of the transactions.

src/ledger/concepts/transactions.md

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+# Ledger: Transactions
+
+> This page is currently a stub

src/ledger/constraints/header-body-split.md

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+# The Header/Body Split in Detail

src/ledger/state-transition.md

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+# The ledger state transition
+
+> This page is a stub.