Add docs and snippets for Sharing Configuration Between Test Suites

[tresat]

Per the discussion on #20831, we want to add a section the test suites documentation to explain how to best configure multiple test suites. Some options to demonstrate are:

Duplicate it (NO EXAMPLE NEEDED)
If it truly applies to all, use all (HIGHLIGHT EXISTING EXAMPLES)
If you have exceptions, make a trade-off between:

Duplicate it all again
Extract the common configuration to a block and apply that to the right elements
Use all, but filter things out with an if

Explain the tradeoffs of these different options in terms of level of declarativity and when each should be recommended

[tresat]

Also update #19870 and this SO question with guidance, once this section is available.