New Feedback form and Dark theme not available in API

[pepinho24]

Describe the bug
The API generated by DocFX have a different rendering and templates. Currently, the new feedback form and Dark theme are not available or tested for them. Example: https://docs.telerik.com/blazor-ui/api/

Additional context
The issue might be mitigated if the _assetsAPI folder is shared between products and moved to docs-seed, e.g. issue #230

The issue is divided into two parts:

1. Add dark theme to Api sections that use DocFX #235
2. Add feedback form to Api sections that use DocFX #236

[dessyordanova]

The WinForms API uses the old Feedback form as well: https://docs.telerik.com/devtools/winforms/api/