doc(all): add Proost logo

kernel/src/lib.rs

+#![doc(
+    html_logo_url = "https://gitlab.crans.org/loutr/proost/-/raw/48-first-release-preparations/docs/media/logo.png"
+)]
+
 //! A kernel for the calculus of constructions.
 //!
 //! Terms can be built with functions from the [`term`] module. This module also provides essential

parser/src/lib.rs

+#![doc(
+    html_logo_url = "https://gitlab.crans.org/loutr/proost/-/raw/48-first-release-preparations/docs/media/logo.png"
+)]
+
 //! Fast parser for λ-terms and commands using pest.
 //! Provides functions to parse files and single commands.
 

proost/src/main.rs

+#![doc(
+    html_logo_url = "https://gitlab.crans.org/loutr/proost/-/raw/48-first-release-preparations/docs/media/logo.png"
+)]
 #![feature(let_chains)]
 
 mod error;