updating documentation links

[Simon Michael]

Hi Craig.. I just noticed the new manual (ledger3.texi) is filling out nicely - thanks! (and to anyone else involved). I 
tweaked the doc links at http://ledger-cli.org to highlight the preferred release and manual.

I still link ledger 3's old manual (ledger.texi) because last I checked it had some updates over the ledger 2 version. 
But I'm about ready to drop it from the site - ok with you ? Hopefully any remaining content will be salvaged and that 
file deleted.