Add guidance for two-tone icons

[joshblack]

We currently support two-tone icons in a couple of situations, this issue is to add information about these to our authoring docs in addition to adding development docs for how to style them.

[stale]

We've marked this issue as stale because there hasn't been any activity for a couple of weeks. If there's no further activity on this issue in the next three days then we'll close it. Thanks for your contributions.

[stale]

As there's been no activity since this issue was marked as stale, we are auto-closing it.

[stale]

We've marked this issue as stale because there hasn't been any activity for a couple of weeks. If there's no further activity on this issue in the next three days then we'll close it. You can keep the conversation going with just a short comment. Thanks for your contributions.

[stale]

As there's been no activity since this issue was marked as stale, we are auto-closing it.

[stale]

We've marked this issue as stale because there hasn't been any activity for a couple of weeks. If there's no further activity on this issue in the next three days then we'll close it. You can keep the conversation going with just a short comment. Thanks for your contributions.