Adds ProdSecEng section for integrating product security requirements

This MR creates a handbook page to document the way that the Product Security team team plans, develops, and maintains functionality required by the GitLab Product Security sub-department.

This relates to this Product Security Engineering Team repo template MR. They are meant to be reviewed and merged together.

Once this is merged, we will need to remove the Automation Request template and update any relevant handbook sections.

See also:

• https://gitlab.com/gitlab-com/gl-security/security-department-meta/-/issues/1783+
• https://gitlab.com/gitlab-com/gl-security/product-security/product-security-engineering/product-security-engineering-team/-/issues/83+
• Follow-up from "Draft: Formalize the Security D... (#282), which this process will likely influence and be influenced by

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