Favicon in website docs missing

[yochem]

The current web pages for the docs do not have a icon in the tab bar. This can be fixed in three ways:

1. Like all pages in the hammerspoon.github.io repo, link to /images/hammerspoon.ico. Pro: easiest. Con: favicon doesn't show in when building docs from this repo.
2. Copy hammerspoon.ico from the website repo to this repo. Pro: Local docs build includes icon. Con: unnecessary icon in this repo.
3. Rename /images/hammerspoon.ico in the website repo to /favicon.ico. Browser automatically use favicons in the root of a website if no icon is specified. Pro: expected location. Con: relies on browser.

Let me know which option you like most! I'll submit a PR for it.