Update translation instructions in the Contribuotrs' Guide

Update translation instructions in the Contribuotrs' Guide

- remove explanation for converting old translations that didn't use
@translationof, as there are probably no longer such docs around;
- really explain committishes stuff;
- remove last traces of the old Documentation index in HTML and the
switch to the new website in Texinfo;
- link to new CG sectiosn about Git usage;
- explicitly tell translators the freedom they may want to take w.r.t.
partial translation of a file or taking distance from the text in
English, using Community section of the website as an example.