Documentation Improvements

Here is a brainstorm of topics that should be included in documentation improvements:

• More detailed documentation on creating new extensions
  • Document differences between the use of AsdfType and CustomType
  • Version assignment of new ASDF types and schemas
  • What does the supported_versions attribute mean and how should it be used
• How external schemas should be packaged
• Registering external extensions using setuptools entry points
• Testing externally provided schemas using the new pytest plugin provided by ASDF
• Better documentation on defining tag and url mappings
• Explanation of why tree data can't (and shouldn't) be accessed after context handler goes out of scope
• Overview of how ASDF-in-FITS works (including documentation of new ASDF HDU)
• Documentation of standalone tool functionality
• Explain new options for warning control
• Give examples of the new external array tag

This probably belongs in the standard documentation:

• Detailed explanation of schema versioning and associated behavior
• Behavior expected when overriding existing tag implementations