Add a build timestamp in the footer of the website

[Aratz]

The website is rebuilt with the latest pipeline info and docs regularly, but it is unclear exactly when. After a release I often get questions about why the pipeline docs have not been updated.

It would be nice to see when the website was build last (e.g. in the footer of every page), to see if there is anything abnormal going on.

[mashehu]

Good idea, I will give it a quick go

[Aratz]

Awesome, thank you!