docs: add page that contains the API docs of the Error package

Author: threeal

docs/error/index.rst

@@ -0,0 +1,15 @@
+Error Package
+=============
+
+A C++ package that provides utilities for error handling.
+
+.. doxygenfunction:: error::make
+
+.. doxygenfunction:: error::format
+
+.. doxygenstruct:: error::Error
+   :members:
+
+.. doxygenfunction:: error::operator==
+
+.. doxygenfunction:: error::operator!=

docs/index.rst

@@ -9,3 +9,11 @@ A comprehensive collection of `C++`_ utility packages.
 .. _build_status_badge: https://github.com/threeal/cpp/actions/workflows/build.yml
 
 .. _C++: https://isocpp.org
+
+Packages
+--------
+
+.. toctree::
+   :maxdepth: 1
+
+   error/index.rst