#5048 Add a warning if translated page is out-of-date

[Federico Bruni]

As suggested here:
https://lists.gnu.org/archive/html/lilypond-user/2016-11/msg00736.html

We should warn the user if a translated page is not up-to-date; and suggest reading the english page (especially if last translation is very old).

The script which checks if a translation is out-of-date is scripts/auxiliar/check_translation.py.
The problem is hacking texi2html to print the warning if the page is out-of-date...

[Federico Bruni]

type Build is more appropriate