TOC

I think that all TOC should be generated automatically. The `TableOfContents` component creates the TOC of the page based on headers (`h2`, `h3`, `h4`, etc.). I just provided `<section class="secs">` elements for ease of making the TOC.

Currently, I put the TOC for only the Guide page experimentally, but it should be reusable for other pages if needed. And the max depth of the TOC is customizable.

Tweaking styles of the TOC is welcome because I've had nothing on styling yet.

The `guide.hbs` seems to has many changes, but mostly by tweaking indents. Please use `git diff -b` or add `?w=1` in this PR's URL if you want to check the simple diff.

So I hope that this PR resolves #84.