Docs: Update image pinning steps for SAST and Secret Detection

What does this MR do?

• Edit SAST section on analyzer image pinning to reflect new language from !103082 (merged)
• Add image pinning section to Secret Detection docs, since one did not exist

I tested copy-pasting these steps in connorgilbert/sandbox!14. I checked that the tags worked as expected.

Related issues

!103082 (comment 1163153353)

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