Show supported language runtime versions in buildpack specific docs

[sophiewigmore]

Although we list supported runtime versions in the release notes now, it might be helpful to have a list of the supported runtimes for each specific buildpack in the docs pages. This would make it easier for a user to find information about the buildpacks all in one place.

We need to consider how we could keep this list up to date, though.

See this slack thread for background

[fg-j]

Displaying this information could make it into #333