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.

README.md

@@ -1,6 +1,14 @@
 # POD2
 
-Specification can be found at the [`book`](https://github.com/0xPARC/pod2/tree/main/book) directory, a live rendered version at: https://0xparc.github.io/pod2/
-
 ## Usage
 - Run tests: `cargo test --release`
+
+## Book
+The `book` contains the specification of POD2. A rendered version of the site can be found at: https://0xparc.github.io/pod2/
+
+To run it locally:
+- Requirements
+	- [mdbook](https://github.com/rust-lang/mdBook): `cargo install mdbook`
+	- [mdbook-katex](https://github.com/lzanini/mdbook-katex): `cargo install mdbook-katex`
+- Go to the book directory: `cd book`
+- Run the mdbook: `mdbook serve`