[doc] Bump Ledger version to 3.1

It would be great to use texinfo variables to set the current Ledger
version, e.g. @value{Ledger_VERSION_MAJOR} and pass the CMake variables
to texi2pdf via --command='@set Ledger_VERSION_MAJOR ${Ledger_VERSION_MAJOR}'.

Unfortunately texi2pdf will err with:
Argument of @parseargline has an extra }.
<inserted text> 
                @par 
<to be read again> 
                   }
@\subtitle ...epage {@subtitlefont @rightline {#1}
                                                  }
l.153 @subtitle For Version @value{Ledger_VERSION_MAJOR} of Ledger


[ci skip]

0 files changed, 0 insertions, 0 deletions