Generate YAML front-matter for Jekyll

[IdkGoodName]

Currently, DefaultDocumentation only generates plain Markdown document without any YAML front-matter. This can be bothering if you use any SSG like Jekyll to document your library.

Current page generation:

### [X](...)
## [X.Y](...)
# [X.Y.Z](...)

Summary
```csharp
public class Z : α
```
<!-- ... -->

Suggested page generation:

---
title: X.Y.Z
member_type: class
description: >
  Summary
definition: >
  public class Z : α
---
### [X](...)
## [X.Y](...)
# [X.Y.Z](...)

Summary
```csharp
public class Z : α
```
<!-- ... -->

Perhaps even possibly allow setting which properties to add.

[Paillat Laszlo]

With the new feature allowing you to add custom sections, you should be able to add just that. I am still cleaning up some stuff and adding documentation to the api but a plugin example of how to add new features should arrive soon.

[Paillat Laszlo]

I added in the samples an example of a basic custom section, you could use that to prefix all your pages with what you need.