User's Guide update: Section 1.5.7 BgcCrop spin-ups

[slevis-lmwg]

(Please see our contribution guidelines for documentation.)

What sort(s) of documentation issue is this?

• Something is missing.
• Something is (or might be) incorrect or outdated.
• Something is confusing.
• Something is broken.

What part(s) of the documentation does this concern?

• Technical Note (science and design of the model)
• User's Guide (using the model and related tools)
• Somewhere else (e.g., README file, tool help text, or code comment): Please specify
• I don't know

Describe the issue

Update the 1.5.7 BGC-Crop spinup section for the new spinup procedure (AD->SASU->pSASU). Include an example workflow (or maybe a contrib script if one exists). Include discussion of SpinupStability script.

Additional context (optional)