Convert docs pages with tables to textile format

Yesterdaty @parkr and I discussed the way documentation pages in the new site containing tables currently require inline HTML for the tables (Markdown doesn’t have support for a table syntax). This isn’t as easy to maintain as it could be, so this PR will convert all docs pages containing tables over from `.md` to `.textile`, and switch to using [Textile’s table syntax](http://redcloth.org/hobix.com/textile/#tables) instead of inline HTML. It’s not ideal to have some docs pages written in markdown, and others in textile, but hopefully this should at least make it easier to maintain the documentation over time. Maybe downstream it would be worth also converting the rest of the docs (back) to textile so it’s all in a consistent format (I originally ported the docs to markdown when I pulled them out of the wiki—maybe that wasn’t a great idea?)

:construction: At the moment only the "structure" docs page is converted over, the rest will come :soon:. There’s probably also going to be some more CSS tweaking needed given the textile table output doesn’t contain some things the original styles expected like `<thead>`, and `<tbody>` elements, and `<p>` tags inside the table cells… All in good time :grin: