Allow for a catch-all page for compiler/sdk error messages

**Issue description**

Currently compiler/SDK error pages do not have a 'catch-all' route, where errors and diagnostics that don't currently have a page can route to. This page could be a call to action to contribute, or just a general 'sorry, we don't have content for this yet' page. Without this, tools that provide links to error pages, like IDEs or terminals, will generate links that 404, presenting a bad experience to the user. Examples of this situation would be:

* [SDK Errors](https://github.com/dotnet/docs/blob/main/docs/fundamentals/toc.yml#L193-L228)
* [C# Errors](https://github.com/dotnet/docs/blob/main/docs/csharp/language-reference/compiler-messages/toc.yml)
* [F# Errors](https://github.com/dotnet/docs/blob/main/docs/fsharp/language-reference/compiler-messages/toc.yml)

Therefore, I propose that a mechanism for creating such a catch-all page be established. The semantics might:
* require a dedicated page for the page content
* have a new top-level node in the toc.yml as a sibling to `name` and `href`, whose value would be the (relative) path to a catch-all template, and which would be invoked if this route path segment matched but none of the children of this route matched.

**Target framework**

- [x] .NET Core
- [x] .NET Framework
- [x] .NET Standard