Move _assetsApi folder from product repos to docs-seed

[pepinho24]

Is your feature request related to a problem? Please describe.
Currently, the _assetsAPI folder needs to be added to each repo that uses DocFX generated API. Example: https://github.com/telerik/blazor-docs/tree/master/_assetsApi

This causes a discrepancy between the appearance of the API of different products and requires each team to make the change locally should a global change is needed. Example, the new Feedback form and Dark Theme.

Describe the solution you'd like
Moving the folder in docs-seed would allow the changes to be done in one place and be used from all products.