Extend merkletree spec, init SignedPod section, add typos checker in CI (#31)

Extend merkletree spec, init SignedPod section, add typos checker in CI

- extend merkletree spec, converting old hand-drawn diagrams to drawio
  diagrams, and adding new diagrams (related: #6)
- init SignedPod section (related: #2)
- initial draft of the types dictionary, set, array (related: #26)
- add typos checker in CI (and correct the ones that were detected)

Note on drawio diagrams: each image file contains the metadata to edit the diagram in the draw.io website.

book/src/signedpod.md

@@ -1 +1,27 @@
-# SignedPOD
+# SignedPod
+
+A SignedPod consists of the following fields:
+- `mt`: key-values storage, internally it's a [MerkleTree](./merkletree.md) so that we can generate proofs on the key-values
+	- it can only contain [`ValueOf` statements](./statements.md).
+	- the Signer's public key is one of the key-values in the `kvs`.
+- `id`: the Root of the `kvs` MerkleTree
+- `signature`: a signature over the `id`
+
+<br>
+
+![](img/SignedPod.png)
+
+
+Example of SignedPod:
+```javascript
+{
+    id: "05f7a6de...",
+    kvs : {
+        _signerKey: "5e1c0b3c...",
+        idNumber: "94e328a7...",
+        dateOfBirth: 1169909384,
+        socialSecurityNum: "1111"
+    },
+    signature: "68b615f7..."
+}
+```