3.x: [Wiki] Enhance Operator-Matrix generation with anchors and explanations

[akarnokd]

https://github.com/ReactiveX/RxJava/wiki/Operator-Matrix

I've turned the column headers to images rotated 90 degrees so the table doesn't overflow the wiki layout.

The src/test/java/io/reactivex/rxjava3/internal/util/OperatorMatrixGenerator.java should be updated accordingly, plus:

• Add HTML anchors to the rows so they can be navigated to via #methodName
• Add <sup>[N](#note-N)</sup> to absent entries and a list of notes below the table explaining why. For this, the reason mapping has to be added to the generator.

[akarnokd]

Closing via #6857