Add TOCTOU to secure code guidelines

What does this MR do?

Describes the "time of check to time of use", some examples, and how to prevent / defend against it.

It also updates a bit about DNS rebinding, since that is one of the main TOCTOU bug classes we are currently seeing.

Related issues

https://gitlab.com/gitlab-com/gl-security/security-department-meta/-/issues/1285

TODO

• Add code examples
• Add links to MRs

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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  • Style Guide.
• Ensure that the product tier badge is added to topic's h1.
• Request a review based on:
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  • The associated Technical Writer.

If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

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