Avoid `link_to` for non-GET requests.

This has come up in a few PRs I've reviewed recently, so I wanted to add it to the guides:

* Using `link_to` with the `method` option introduces an un-necessary dependency on JavaScript.
* HTML elements—in this case a form or a link—should be chosen for their semantics, not their appearance.
* CSS can be used to make links look like buttons and buttons look like links.