generate-doc: don't mention internal interfaces

[benjamin-hodgson]

internal interface IInternal
{
}

public class Public : IInternal
{
}

Running generate-doc on this code produces output markdown that looks like this:

---
sidebar_label: Public
---

# Public Class

[Constructors](#constructors) • [Methods](#methods)

**Namespace**: [<global namespace>](../index.md)

**Assembly**: Test\.dll

```csharp
public class Public : IInternal
```

### Inheritance

[Object](https://docs.microsoft.com/en-us/dotnet/api/system.object) → Public

### Implements

* [IInternal](../IInternal/index.md)

## Constructors

| Constructor | Summary |
| ----------- | ------- |
| [Public()](-ctor/index.md) | |

## Methods

| Method | Summary |
| ------ | ------- |
| [Equals(Object)](https://docs.microsoft.com/en-us/dotnet/api/system.object.equals) |  \(Inherited from [Object](https://docs.microsoft.com/en-us/dotnet/api/system.object)\) |
| [GetHashCode()](https://docs.microsoft.com/en-us/dotnet/api/system.object.gethashcode) |  \(Inherited from [Object](https://docs.microsoft.com/en-us/dotnet/api/system.object)\) |
| [GetType()](https://docs.microsoft.com/en-us/dotnet/api/system.object.gettype) |  \(Inherited from [Object](https://docs.microsoft.com/en-us/dotnet/api/system.object)\) |
| [MemberwiseClone()](https://docs.microsoft.com/en-us/dotnet/api/system.object.memberwiseclone) |  \(Inherited from [Object](https://docs.microsoft.com/en-us/dotnet/api/system.object)\) |
| [ToString()](https://docs.microsoft.com/en-us/dotnet/api/system.object.tostring) |  \(Inherited from [Object](https://docs.microsoft.com/en-us/dotnet/api/system.object)\) |

IInternal isn't accessible to consumers and so should not be mentioned in the "Implements" section or in the code snippet showing the declaration. This appears to affect both the github and docusaurus outputs.