(GH-10574) Update about_Enum

Prior to this change, the `about_Enum` documentation didn't include
the ability to set underlying base types for PowerShell 6.2 and later.
It also didn't clearly identify the available methods for using
enumerations outside of examples, discuss formatting enumeration values
with format strings, show how to define extension methods, or include
documentation about the limitations of enumerations.

This change:

- Resolves #10574
- Fixes AB#182987
- Reorganizes and extends the documentation to cover missing topics,
  make it easier to discover syntax, and add more examples. This work
  mimics the restructuring of `about_Classes`, but without creating new
  articles.