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.
