website: experiment with badge endpoint (#5423)

If this works out, one should be able to include

    https://openpolicyagent.org/badge/v0.46.1

as an image, say, in a github README, and it'll look nicely.

Furthermore, when 0.46.1 will no longer be the latest version, it'll
turn from green to yellow.

Signed-off-by: Stephan Renatus <stephan.renatus@gmail.com>