Move source into `src` directory.

README.md

@@ -36,5 +36,5 @@ project, first install the following:
 Afterward, you can generate the necessary HTML via:
 
 ```bash
-> find . -name '*.tex' | grep -v preamble | xargs -I {} make4ht -e build.mk4 {}
+> find . -name '*.tex' | grep -v preamble | xargs -n 1 make4ht -e build.mk4
 ```

lakefile.lean

@@ -16,6 +16,7 @@ require «doc-gen4» from git
 
 @[default_target]
 lean_lib «Bookshelf» {
+  srcDir := "src",
   roots := #[
     `Bookshelf,
     `FirstCourseAbstractAlgebra,