[Feature Request] Enable Nullable reference types on solution level

**Is your feature request related to a problem? Please describe.**

Currently docfx `v2` use `#nullable enable` on per file level.
It might be better to apply this settings to solution level.

I know it requires a lot of works to supporting this feature.
But It seems worth putting on the future roadmap.

**Describe the solution you'd like**

Add following settings to `Directory.Build.props`

```xml

enable

```

**Related Issues**
- https://github.com/dotnet/docfx/issues/5515