document new location for source code in github.com/fedwiki

I have devised a new scheme for editing plugin documentation and have tested it on this obscure plugin. Rather than move pages into and out of some local wiki I have running, I just launch a wiki on the plugin repo.

    cd wiki-plugin-calculator
    wiki -p 3010 -d .

This has a few strange effects.

* there is no welcome-visitors, but the default offers recent-changes which works well.
* there is no status/favicon.png so this gets made and must be ignored on commit.
* the wiki-node-server is too quick to label doc pages as plugin: "calculator". I'll put in a fix for that.

I'm feeling the desire to run through all of our plugins and make similar improvements. There might be some workflow optimizations regarding getting a bunch of documentation changes published to npm. I'm open to suggestions.