Docs: Remove link to stale AppSec whitepaper

What does this MR do?

Remove a marketing callout to a whitepaper from the top of the SAST docs. This is for two reasons:

1. The whitepaper itself looks like it is rather old; the mirror of it on GDrive looks to be last updated in 2020 (internal link).
2. I do not like using docs to drive traffic to whitepapers or other marketing assets. This is the only place this whitepaper appears in the docs, suggesting that its presence on the SAST docs is not part of a Sec-wide campaign or similar.

Related issues

None

Author's checklist

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Reviewer's checklist

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If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

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