Support easy, yet precise, documenting of relational operators

C++20 brings a new 3-way relational operator and three classes of comparison result: strong, partial, and weak ordering (cf. epic). We need an easy way to document

1. the class of comparison result that applies to the type: strong, partial, weak
2. which other types this type can be compared with

qdoc should then do the rest.

Example: QByteArray

\class QByteArray
\stronglyordered
\comparesto QByteArrayView, const char*, const char8_t*, ...

This is all a class author should need to do. Output should be something like:

class QByteArray

This class is _strongly comparable_ (link to description).

It can also be compared to <list or table>

<add function documentation for each op in {==, !=, <, >, <=, >=, <=>}, complain if one is missing>

This is for less-than-comparable types. We'll need something similar for equality-comparable-non-less-than-comparable types, such as QRect:

\class QRect
\stronglyequalitycomparable 
\comparesto QRectF

\class QRectF
\partiallyequalitycomparable
\comparesto QRect

Scope for this ticket is to finalize the concept and generate buy-in from the Documentation team and the qdoc maintainers for it.

Acceptance criteria:

• common understanding of the solution between Documentation team, qdoc team and C++ developers, incl. buy-in
• follow-up tasks defined for each of Documentation, qdoc, and C++ teams