Update README to include serverless

[joepeeples]

Important

Do not merge until after serverless source files have been moved to elastic/security-docs.

Contributes to https://github.com/elastic/security-docs-internal/issues/13.

Updates the README in security-docs to include information about serverless.

Preview: https://github.com/elastic/security-docs/tree/internal-13-update-readme-classic

[github-actions]

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[mergify]

This pull request does not have a backport label. Could you fix it @joepeeples? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• v7.x is the label to automatically backport to the 7.x branch.
• v7./d./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.