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.

.github/workflows/typos.yml

@@ -0,0 +1,19 @@
+name: typos
+on:
+  pull_request:
+  push:
+    branches:
+      - main
+
+jobs:
+  typos:
+    if: github.event.pull_request.draft == false
+    name: Spell Check with Typos
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Use typos with config file
+      uses: crate-ci/typos@master
+      with: 
+        config: .github/workflows/typos.toml
+