This adds a new page that covers how to store extra_data on resources

using the old assignment method and the newer merging/patching methods.

It covers usage in detail and links to the specs for more information.

It also talks about access restrictions, including the prefix for

private keys and the impact of APIExtraDataAccessHook.
All resources that use extra_data now link to the new docs. Some of the

older resources that talked about assignment have now had that

information removed in favor of the new page.

Built and read through the new docs. Checked for build issues, spelling

mistakes, and all links.