Find a way to record in docs the release version for new features

[claremacrae]

Based on conversation in #716.

current thinking is to pop a placeholder like x.y.z when documenting a new feature.

[claremacrae]

I've started on this in #731.

For now, the X.Y.Z markers will need to be manually updated before each release. I'll automate that later on.

[claremacrae]

This is now released - in 1.7.0.