Update bgc spinup section

[slevis-lmwg]

Description of changes

Start with the issue #3975

Specific notes

Contributors other than yourself, if any:

CTSM Issues Fixed (include github issue #):
Resolves #3975

Are answers expected to change (and if so in what way)? No

Any User Interface Changes (namelist or namelist defaults changes)? No

Does this create a need to change or add documentation? Did you do so?
This PR updates documentation.

• I generated new figures in /glade/work/slevis/git/ans_chging_tags/tools/contrib. Use them to replace all 'placeholder.png' in my .rst file mods.
• Include example workflow; so far I have looked back at my spin-up workflows from 2-deg I-case spinup and historical baseline for PPE with BNF tag (ctsm53017_f19_BNF_[AD, SASU, pSASU, hist]) NCAR/LMWG_dev#88, 1-deg I-case spin-up and historical to test new paramfile for CESM3 (ctsm53019_f09_BNF_[AD, SASU, pSASU, hist]) NCAR/LMWG_dev#89, 1-deg I-case spin-up and historical to test new CLM6 paramfile (ctsm53026_BNF_[AD, SASU, pSASU, hist]) NCAR/LMWG_dev#91, but I should use vetted simulations from WhatsNewInCTSM5.4, such as ctsm5.4.CMIP7_ciso_ctsm5.3.075_f09_124_ [AD, SASU, pSASU, HIST] NCAR/LMWG_dev#124
• Do we have an example of SASU successfully bringing a pretty-far-along case to equilibrium? In clm5 we recommended AD for that, which makes more sense, and which also automatically reseeds dead plants...
• Include discussion of the SpinupStability script

[slevis-lmwg]

@olyson could you review the spinup section that comes right after yours? I tried to simplify this section relative to the clm5 version since now there's the extra step of SASU.