Apply field-list and admonition handling to classes and unions

[jakobandersen]

Most declarations use the description() method to render the brief and detailed descriptions. That function which pulls up field-lists and admonitions from the detailed description, such that Sphinx can process the field-lists.
But classes and unions render their content through visit_compounddef() which didn't process the detailed description.
This PR factors out the handling so both methods can apply the handling.
This incidentally also means that classes and unions now get affected by breathe_order_parameters_first which, despite its name, determines the order between field-lists in general (including template parameter documentation) and admonitions.

Fixes #761.

As test case:

/// @brief Type
/// @tparam E the type parameter
template<typename E>
using Type = int;

/// @brief Concept
/// @tparam E the type parameter
template<typename E>
concept Concept = true;

/// @brief Var
/// @tparam E the type parameter
template<typename E>
int Var = 42;

/// @brief Function
/// @tparam E the type parameter
template<typename E>
void Function() {}

/// @brief Class
/// @tparam E the type parameter
template<typename E>
class Class {};

/// @brief Union
/// @tparam E the type parameter
template<typename E>
union Union { int i; };

The concept declaration seems to no be handled correctly by Doxygen, so it should be checked at a later stage.

[2bndy5]

Here are some screenshots from this PR's build docs CI artifacts

For template classes

For template functions

---

I think it's safe to say this solution works as intended

[jakobandersen]

Great, thanks for testing!