DOC: Update xdist to provide more speed-up options and context (#48958) (#48535)
👋 I was working on a more elaborate solution to the question in #48535 when @mroeschke made #48958 and closed it. So I tried to incorporate his work into mine to see if you are still interested in a more verbose solution. Things that might be interesting here are:

- mentioning that pytest-xdist is in our environment and doesn't need to be installed
- explaining the other speed up options which were available in the original script
- explaining the -r summary flag and stating it's optional
- showing people how to set a seed on Windows and Unix machines in case they need help (this might be redundant, I wasn't sure).

Screenshot for illustration:
<img width="728" alt="Screenshot 2022-10-07 at 10 36 41" src="https://user-images.githubusercontent.com/6564007/194512399-897fdfef-4432-4400-a7c0-b9c71a781d94.png">


Please don't hesitate to close this if it's overkill at this point.
When demonstrating skipping tests, could we promote using e.g. `-m "not slow"` instead of `--skip-slow`? I think using `-m "not ..."` will be more flexible/scalable in the instance where we add more marks in the future and accomplishes the same test skipping. thx @noatamir 