Add DocFx metadata element

[Cloud-Awesome]

e.g. something to replace this:

var docfxMetadata = 
    "---" + Environment.NewLine +
    "uid: this_is_a_tester" + Environment.NewLine +
    "---";

new MdDocFxMetadata(key, value)

[Cloud-Awesome]

This maybe needs a yaml generator class too...

e.g.

---
documentType: tester
seeAlso:
  - uid: this_is_a_uid
    name: New page
  - uid: this_is_another_uid
    name: Second page
---

Consider whether this is a new class within the md maker, or a separate project for yml..?