Updates to Avatar docs

[jennaingersoll]

Resolves a number of issues regarding avatar docs:

• Removed obsolete/incorrect information about avatars from docs
• Updated our Avatar Standards
• Reorganized the docs so users can figure out how to change, create, host an avatar more easily
• Made it so that "create your avatar" implies many ways to do it; app-specific instructions moved into "tutorials"
• Removed references to attachments - changed to wearables
• Added links to community plugins to create avatars (right now, links to Menithal's Blender plug-in, but easy to add more)