Generate error messages documentation from enum

[adamconnelly]

At the moment there's a bit of duplicated effort between the CompilerMessageId enum and the Compiler Messages documentation. We could probably write a simple tool to extract the doc-comments from the enum, and generate a markdown page based on it.

Here's an example of what one of the enum members looks like currently:

/// <summary>
/// The `slist` type is deprecated, and should no-longer be used. Please use the `string`
/// type instead. For example:
/// ```
/// struct User {
///   slist Username
/// }
/// ```
/// </summary>
SlistDeprecated = 204,

There might be a simpler way to do this, but we can definitely use Roslyn to parse the file and grab the comments out, so it shouldn't be difficult to do. Alternatively maybe we ought to just investigate hosting the API docs for the C# code somewhere as a class reference and linking to the enum.