Add build information to the footer and HTML header

[nuest]

To avoid any confusion, we can add a little piece of information to the footer of the register, in the HTML rendering: the build date and the used version of the R package, also the used git hash from the register... the gitr package should easily give us the commit hash.

Mockup:

@angelina-momin What kind of information would you find useful here?

In addition, we can also add that information in the HTML head, see https://html.spec.whatwg.org/multipage/semantics.html#standard-metadata-names

<meta name=generator content="codecheck 0.7.0 based on register commit abc123">