Automatically detect missing doxygen comments (#1226)

Fixes #1216

CONTRIBUTING.md

@@ -24,6 +24,7 @@ git config --local core.hooksPath .githooks
 The pre-commit hook requires `clang-format >= 17.0.0` and `cmake-format` to be installed on your machine.
 `clang-format` can be installed using `brew` on macOS and default package manager on Linux.
 `cmake-format` can be installed using `pip`.
+The hook will also attempt to automatically use `doxygen` to verify that everything public in the codebase is covered by doc comments. If `doxygen` is not installed, the hook will raise a warning suggesting to install `doxygen` for future commits.
 
 ## Git commands
 This sections offers a detailed look at the git commands you will need to use to get your PR submitted. 
@@ -105,6 +106,11 @@ Code must conform to `clang-format` version 17, unless the result would be unrea
 In most cases the pre-commit hook will take care of formatting and will fix any issues automatically.
 To manually format your code, use `clang-format -i <your changed files>` for C++ files and `cmake-format -i <your changed files>` for CMake files.
 
+## Documentation
+All public namespaces, classes and functions must be covered by doc (`doxygen`) comments. Everything that is not within a nested `impl` namespace is considered public.
+
+> **Note:** Keep in mind that this is enforced by Clio's CI and your build will fail if newly added public code lacks documentation.
+
 ## Avoid
 * Proliferation of nearly identical code.
 * Proliferation of new files and classes unless it improves readability or/and compilation time.