bug#50880: Add "Contributing to Emacs" help command (see C-h C-h)

[Gregory Heytings]

> > > I suggest:
> > >
> > > C-h g -> About the GNU project
> > > C-h G -> Emacs copying permission (GNU General Public License)
> > > C-h C-c -> Contributing
> >
> > Sounds good to me.
>
> Here is another problem:
>
>  TAB Show all installed manuals
>
> should be
>
>  C-i Show all installed manuals
>
> because the mnemonic key is 'C-i'.

But "Show all installed manuals" is on C-h i, isn't it?  Has this changed 
very recently?