Allow OpenAPI 3 "schemas" specification in docstrings (like "definitions" on Swagger 2.0)

One nice thing with flasgger and Swagger 2.0 when using docstrings is that you can specify any `definitions` (mostly) specific to one route/path alongside its responses and parameters that may use those.
With OpenAPI 3, those `definitions` moved to `components/schemas`, but it would be great to still specify them in the docstrings of the various routes/paths. There could be a `schemas` entry that we specify there and that gets merged up to `components/schemas` just like `definitions` is for 2.0.

Start by reviewing the existing docstring handling for Swagger 2.0 `definitions` and the OpenAPI 3 `components/schemas` path. Confirm that route-level `schemas` entries merge into `components/schemas` while preserving the behavior already provided for `definitions`.

Written by the indexing model from the issue text.