Landing pages and help for the HP

[Don Stewart]

http://hackage.haskell.org/platform/new/

Has two new changes:

 * Per-platform "landing" pages, where we can describe the packages
for each system:

    + http://hackage.haskell.org/platform/new/mac.html
    + http://hackage.haskell.org/platform/new/windows.html
    + http://hackage.haskell.org/platform/new/linux.html

And the "Help" link, which goes to a wiki where we can document any
issues, as well as suggesting tutorials and so forth.

If you maintain one of the installers, patches can be made to the
landing pages via the markdown files in the darcs repo:

  darcs get http://code.haskell.org/haskell-platform/

in the download-website/new/ directory.

Also, you can improve the installation instructions on the wiki pages:

  http://haskell.org/haskellwiki/Haskell_Platform

Thanks everyone so far for the feedback.