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.

src/merkletree.rs

@@ -31,7 +31,7 @@ pub struct MerkleTree {
     // kvs are a field in the MerkleTree in order to be able to iterate over the keyvalues. This is
     // specific of the current implementation (Plonky2's tree wrapper), in the next iteration this
     // will not be needed since the tree implementation itself will offer the hashmap
-    // functionallity.
+    // functionality.
     kvs: HashMap<Hash, Value>,
 }