QDoc: Implement COMMAND_COMPARES

This is an implementation task resulted from QTBUG-104110.

In summary;

Introduce a new command, `\compares`, to QDoc. The command shall take two arguments
1) the comparison category that applies to the result of comparing the type being documented with the type(s) in the second argument. This argument shall be one of the following:

  `strong`
  `partial`
  `weak`
  `equality`

2) A list of types this type can compare to with the comparison category given in the first argument.

QDoc shall generate a statement as part of the type's documentation, e.g:

    `This type compares strongly with QFoo, QBar, and QBaz.`

Multiple occurrences of the command must be permissible, to allow e.g. documenting that a type T compares strongly with type Y, and also that type T compares weakly with type Z.