Add introduction to OpenAPI doc

What does this MR do and why?

This adds an introduction to the OpenAPI documentation YML file like suggested in !181715 (comment 2356886928)

References

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• Related epic: &8926

How to set up and validate locally

• To generate the OpenAPI documentation, run bin/rake gitlab:openapi:generate
• To check that the documentation is up-to-date, run bin/rake gitlab:openapi:check_docs

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