Improve versatility and robustness of `cpptype`, and document it in the user guide

This MR improves the implementation and interface of cpptype, adds a number of tests, extends the STL API reflections using it, and introduces a user guide section on API reflection.

lang.cpptype

• Fix correct cloning of CppType, as required for (de)constify, by keeping and passing on the raw template parameters
• Customize the template formatter to reject superfluous template arguments
• Have cpptype return a CppTypeFactory object which is used to instantiate class templates, and apply const and ref qualifications
• Introduce the convenience subblass CppClass of AugExpr to simplify the definition of API reflections for C++ classes

lang.cpp.std

• Update reflections of C++ STL classes to use the new features of cpptype

Composer

• Remove the undocumented cpptype method from the composer, to avoid confusion about its name

Documentation

• Add a user guide section on cpptype and API reflection using CppClass
• Add myst-nb as the MyST markdown engine in order to parse and execute Jupyter and text-based notebooks