docs: Readd block header

docs/Blocks/header.md

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+import APITable from '@site/src/components/APITable';
+
+# Block structure
+
+In StarkNet, the block is defined as a list of transactions and a block header.
+
+## Block header
+
+The following fields define the block header:
+
+<APITable>
+
+| Name                     | Type                     | Description                                                                               |    Implemented     |
+| ------------------------ | ------------------------ | ----------------------------------------------------------------------------------------- | :----------------: |
+| `parent_block_hash`      | `field element`          | The hash of this block's parent                                                           | :heavy_check_mark: |
+| `block_number`           | `integer` \| `"pending"` | The number (height) of this block                                                         | :heavy_check_mark: |
+| `global_state_root`      | `field element`          | The hash of this block's parent                                                           | :heavy_check_mark: |
+| `sequencer_address`      | `Ethereum address?`      | The Ethereum address of the sequencer submitting the state update that creates this block | :heavy_check_mark: |
+| `block_timestamp`        | `timestamp`              | The time the sequencer created this block before executing transactions                   | :heavy_check_mark: |
+| `transaction_count`      | `field element`          | The number of transactions in a ablock                                                    | :heavy_check_mark: |
+| `transaction_commitment` | `field element`          | A commitment to the transactions included in the block                                    | :heavy_check_mark: |
+| `event_count`            | `field element`          | The number of events                                                                      | :heavy_check_mark: |
+| `event_commitment`       | `field_element`          | A commitment to the events produced in this block                                         | :heavy_check_mark: |
+| `protocol_version`       | `integer`                | The version of the StarkNet protocol used when creating this block                        |                    |
+| `extra data`             | `field element`          | Extraneous data that might be useful for running transactions                             |                    |
+
+</APITable>
+
+:::info commitments
+The commitment fields transaction_commitment, event_commitment are the roots of a hieght 64 binary Merkle Patricia tree. The leaf at index $i$ corresponds to the hash of the $i'th$ event/transaction.
+:::
+
+## Block hash
+
+The block hash is defined as the Pedersen hash over the header's elements.
+
+$$
+\begin{aligned}
+h(B)=h(&\text{block\_number}, \text{ global\_state\_root}, \text{ sequencer\_address}, \text{ block\_timestamp}, \\
+&\text{transaction\_count}, \text{ transaction\_commitment}, \text{ event\_count},\\
+& \text{event\_commitment}, 0, 0, \text{ parent\_block\_hash})
+\end{aligned}
+$$
+
+Where $h$ is the [Pedersen](../Hashing/hash-functions#pedersen-hash) hash.
+
+:::note placeholders
+Zeroes inside the hash computation of an object are used as placeholders, to be replaced in the future by meaningful fields.
+:::