content: Document new dimensions

[jmooring]

For the most part these are just stubs...

Site config:

• [roles]
• [versions]
• defaultContentRole
• defaultContentRoleInSubdir
• defaultContentVersion
• defaultContentVersionInSubdir

Site methods:

• Version
• Versions
• Role
• Roles
• Dimension

Front matter:

• languageDelegees
• languages
• roleDelegees
• roles
• versionDelegees
• versions

Page methods:

• Rotate

Other:

• Dimensions (concept). For now place this under content management, and later move to the concepts section in the new structure. Consider using isometric cube to visually describe the concept.

[bep]

I will try to "narrow down" and set the API in the next few days. I will also try to create a test version of the docs site that demonstrates the core concepts in #3117

The perhaps most useful new method in all of this is Page.Rotate which is like Page.AllTranslations but for all of language, version and role.