DOC: Update xdist to provide more speed-up options and context (#48958) (#48535)

[noatamir]

👋 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:

Please don't hesitate to close this if it's overkill at this point.

[phofl]

I like the clarifications, couple of these things weren't clear to me either when getting started with pandas

[mroeschke]

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.

[jorisvandenbossche]

I think this is a nice addition (it's certainly good to document how to use the markers)

[phofl]

thx @noatamir