Support Overlays standard

Nobody has claimed this yet.

There is a reasonably new formalised standard out called "Overlays - https://github.com/OAI/Overlay-Specification/blob/main/versions/1.0.0.md 2024-10-17 .

Currently, I can generate my openapi definition from the code, then add a few tweaks to it, commit this to a documentation site, but the second the source code changes, then one has to manually add in the tweaks again, repeat this process for every update.

This standard helps a lot with that, since now, using something like - https://github.com/lornajane/openapi-overlays-js - then any additional changes can be kept in a seperate file, then added back in when we want to add it.

Currently, if there is an issue with the generator, or you just want to add in some new properties to the auto-generated code, then you either have to fix this repo or then use a 3rd party tool to then join the two back together again.

Might be nice to add in an overlay merging step, but if this is outside the scope of this project, then that's also fine since adding more features (even if pulling in a library for most of it) does distract from the main scope of this project.

Or probably one to make a plugin that is compatible with this library and then can be added to the [Ecosystem page](https://github.com/marshmallow-code/apispec/wiki/Ecosystem)?

Start by reading the linked Overlays 1.0.0 specification and the openapi-overlays-js project, then inspect how apispec generates and exposes its OpenAPI definition. Clarify whether the desired outcome is an integrated overlay-merging step or a separate compatible plugin listed on the Ecosystem page.

Written by the indexing model from the issue text.