Update "Running Stock Synthesis"

[iantaylor-NOAA]

I'm assigning this to myself to work on a few changes which could be helpful:

• Add a link to the SS3 website and/or manual where more detailed information is available
• In the section on model tuning, update to match the current advise in the PFMC SSC best practices doc as the quoted text is being revised for 2025 (needs to wait for the 2025 document to be finalized)
• Under "Discard ratios and mean body weight" add a description of the $discard_tuning_info and $mnwgt_tuning_info tables that get returned by r4ss::SS_output() and reference the discussion of how to use these tables in add function, or expand SS_tune_comps(), to tune discard rates and mean body weight r4ss/r4ss#498 (comment) (and earlier comments on that issue).
• Work with @e-perl-NOAA to update the SS3 User Manual or website with information that is useful for all users and consider linking to that source in addition to or instead of keeping the info in the handbook.