Namespace documentation

[kostiantxn]

We should consider adding support for documentation for namespaces.

Unfortunately, in C# namespaces cannot have XML documentation. As an alternative, we could use something more convention-based. For example, define a file-scoped Namespace class and attach documentation to it:

namespace Summary;

/// <summary>
///     The core <c>Summary</c> namespace which defines pipelines
///     for parsing C# source code into <see cref="Doc"/> objects.
/// </summary>
file class Namespace;

We could also ignore C# restrictions, and attach documentation to the namespace regardless:

/// <summary>
///     The core <c>Summary</c> namespace which defines pipelines
///     for parsing C# source code into <see cref="Doc"/> objects.
/// </summary>
namespace Summary;

Given that we use static syntax analysis, we should still be able to extract the leading trivia for the namespace and convert it into proper namespace documentation.