Doc updates

[gjtorikian]

Resolves #1024.

This brings the Changer package tutorial a bit up-to-date. A few things to note:

• I feel pretty strongly about keeping the general package creation docs separate from the "creating your first package!" docs. One is a reference, the other is a tutorial. In the interest of not keeping a giant page, I split the two parts.
• I wouldn't merge just yet. I'll still likely need to update theme-variables.md and the companion "Your first theme!" docs
• Not that my code is perfect and marvelous, but it might be time to use panda-docs (or some other module) to handle the creation of a mini-docs site, complete with a TOC, anchors for headings, and all that jazz

/cc @benogle @mcolyer

[mcolyer]

I'd really rather see the guide/tutorial more intertwined than they are currently which is why I put them together. I think organizing our documentation around tasks (get started, create a theme, create a package) makes it simpler to find what you're looking for if you're new.

That said I'm also not the one putting the time in on it, so ultimately I leave it up to you.

[benogle]

I agree with @gjtorikian. Having the example jammed in there makes it hard to find. We can link to the example in the beginning of the reference/info section of the package doc.

[benogle]

Should this be changer:magic?

[gjtorikian]

Good catch !

[benogle]

Other than my one comment, I like this

[gjtorikian]

@benogle Rather than refer to local files, I ended up linking out to GitHub's cloud asset host: https://github.com/atom/atom/pull/1052/files#diff-6b8aea9919633dfccd46cb74e4735766R330