mark "TODO"-blocks with admonition markup

(also see https://codeberg.org/openpgp/notes/issues/3)

book/source/03-cryptography.md

@@ -1,9 +1,10 @@
 # Cryptographic concepts/terms
 
-```
-Lars suggests that we should have a chapter where we introduce cryptographic terms that we use, and give short definitions (without getting into how specific algorithms work)
+```{admonition} TODO
+:class: warning
 
-This would be a good place to introduce visualizations for cryptographic primitives
+- Introduce cryptographic primitives/terms at a very superficial level
+- Introduce visualizations for cryptographic primitives
 ```
 
 ## Public-key cryptography
@@ -45,17 +46,24 @@ OpenPGP makes heavy use of public-key cryptography. However, for historical reas
 
 So when reading the RFC, or other documentation, you will encounter the term "secret key," instead of the more common "private key."
 
-### Symmetric encryption
+## Symmetric encryption
 
 https://en.wikipedia.org/wiki/Symmetric-key_algorithm
 
-[TODO: visualization?]
+```{admonition} TODO
+:class: warning
 
-#### Symmetric cryptography in OpenPGP
+- visualization?
+```
+
+### Symmetric cryptography in OpenPGP
 
 Symmetric encryption is a core concept in OpenPGP. It usually comes up involving the term "session key."
 
 "Session keys" in OpenPGP are symmetric cryptographic keys.
 
+## Hashing
+
 ## Cryptographic signatures
 
+## AEAD