feat: docstring parsing

[artcg]

Closes #203

I decided to go with an approach where the user can specify their own yaml instead of going with a more complex parsing approach.

This has the upsides:

• https://github.com/flasgger/flasgger (openapi for flask) does something similar so users coming from there will be able to migrate easily
• enables user to modify the yaml however they want, no restrictions
• openapi3 compatibility straight away

this has the downside:

• corrupted yaml could be hard to debug for beginner users

Implementation notes:

• if yaml parsing fails, raises a warning and then ignores the yaml
• if user uses doc decorators, it will use those first and then the yaml second

For the future:

Somebody could add functools.wraps to the doc decorators if they would like to use them in conjunction with the docstring

I am not using those decorators so didnt look into that, actually the main point I wanted this feature was to avoid having to use the decorators

Heres a usage example:
https://i.imgur.com/ViynuNY.png
https://i.imgur.com/oORSqRa.png

Feedback welcome, tagging people who could be interested @denisovkv @ruckc @ioggstream

[denisovkv]

Awesome feature! Suppose it should support every eligible yaml not only simple struct with one response and description as in example, am I right?

[artcg]

Yep, could be anything in the path operation spec :)

[denisovkv]

Looks good for me