Move conceptual documentation to the docs repo

[sdwheeler]

This is one of several issues for managing the documentation in preparation for publishing on docs.microsoft.com.

Move the following articles to the docs repo

• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/PowerShellBestPractices.md
• https://github.com/PowerShell/PSScriptAnalyzer/blob/master/ScriptRuleDocumentation.md

Update the README.md

• Most of the content of the README should be in a usage document in the docs repo.
• Content specific to using or building the source code and the rules of the repo belong in the README.
• Update the README to point to the new documentation online.