doc URL fixes!

Fixes URLs for documentation, so people can actually find the documentation. There is a comment in /urls.py that "/docs are an Apache rewrite", so some of my changes could be rendered perhaps unnecessary (or require a different approach) if the Apache directive was updated.

I did not fix some URLs in /apps/jetpack/fixtures/core_sdk.json , not knowing whether this was just historical (or what it was for), whereas with /apps/jetpack/templates/js/first_addon.js , I changed the URLs as it looked like the widget might still work today, though I didn't test it (and it also used the "new" keyword in front of Widget which I guess is no longer necessary).

Even if this is imperfect, I really suggest addressing this quickly as the whole site is kind of meaningless if one can't find the docs (though "Submit to AMO" is another critical item to make the site worthwhile but at least one can still test without it if one can find the docs).