Docs: Edit Secret Detection intro, clarify detection scope

What does this MR do?

Update the introductory framing of Secret Detection to:

• Simplify some sentences.
• Remove emphasis on passwords, which are not the focus of our current detection capability.
• De-emphasize the underlying scan engine, which has confused people about the source of rules and the scope of our support responsibility.
• Highlight Ultimate-only features.
• Remove an out-of-date screenshot. I could replace it but didn't feel it was providing a ton of value.
• Fix the name of the CI/CD job (secret_detection, not secret-detection).

Those changes were along the way to the main event, which was an update to the section addressing what secrets we detect. I've changed this section by:

• Explaining what well-identifiable secrets are (though not using that specific term, since it isn't widely adopted).
• Noting that we only search for passwords in specific places, not everywhere.
• Moving the "if you want to contribute" text to a separate section and updating it to not point to a closed issue, and also cross-promoting our partner program for automatic response.

Related issues

I started the MR because of an uptick in questions about why we aren't detecting passwords when people are trying to test Secret Detection. See Slack (team members only).

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