Added a lookup form to the homepage, a brief definition, added Home link...

This pull request:

* Adds a lookup form to the homepage that redirects the user to the results on `client.webfinger.net`.
* Adds a brief definition of Webfinger to the homepage above the RFC.
* Adds a Home link to nav as the first thing
* Makes minor tweaks to Markdown for simplicity, and copy for readability