As an hobby I wrote a game in JavaScript and would love to publish it to Facebook, can you write a Facebook application using pure HTML and JavaScript?
Yes, though your app may not be able to do much on Facebook. Have a look at http://developers.facebook.com for documentation.
Yes. Absolutely. In fact, one of the options for developing a Facebook application is an embedded frame pointing to your website which can be whatever you want, including straight HTML web serving.
There are two officially sanctioned libraries for accessing Facebook information and one of them is Javascript based.
You can do quite a bit on Facebook with just Javascript/HTML. There's a lot of possibilities using services outside of your own site (think Flckr, Google Ajax API etc...)
Facebook applications are pleasantly simple to develop.
The documentation is indeed at: http://developers.facebook.com/
I would also note that traffic with regards to Facebook, things can spin up really quickly. There are stories of students creating applications which quickly scaled to millions of users. I'm not saying this is likely to be the case for you, but it is can be an effective and a highly viral distribution platform.