Move API docs to an api directory

**Note: this is targeted at gh-pages, not master**

Landing this will move the generated `yard` docs from the root of the gh-pages site https://rapid7.github.io/metasploit-framework to https://rapid7.github.io/metasploit-framework/api .

This is necessary to have a more user-friendly landing page at https://rapid7.github.io/metasploit-framework (iow, the internal MetaKitty project).

/cc tdoan-r7

It also updated the API docs to the latest on upstream-master.

I'm talking to @bturner-r7 and @cdoughty-r7 about the best way to automate this (probably a Jenkins task).

Once this lands, I'll start redirecting https://dev.metasploit.com/api to here.