Consider including date/time or rosdoc2 version in generated docs

[christophebedard]

For ros2/ros2_documentation#5050, I was looking at docs for different packages @ different distros and I was seeing differences in outputs. This is most likely because the docs were built at different times, so they used different versions of rosdoc2. I was trying to figure out when the docs were built (Rdoc/etc. jobs) to then figure out if they were using a recent or old version.

I think it would be helpful to include the date the docs were generated and/or the rosdoc2 version somewhere in the generated docs. It could even be sort of hidden. I've looked a bit and I don't think this is the case already, but I could be wrong.

Just a suggestion!

[christophebedard]

If this makes sense and we pick a specific location for it, I can look into implementing it.