doc: Add Sec Section development docs

What does this MR do and why?

Adds a dedicated sectionsec Feature Development section to docs.gitlab.com's development docs.

The goal is to add a new consolidated place for development docs to extract from the currently distributed context of the handbook and various project READMEs.

The current content is almost a direct port of the Secure Technical Documentation section since it should belong within development docs, not the handbook. I cleaned up some word-choice but most of the content is as-is.

See #370011

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