[NaCl SDK Docs] Update PPAPI docs to use Chromesite-friendly URLs.

* Chromesite URLs must end in .html, unlike devsite.
* Non-doxygen docs are no longer versioned, so {{pepperversion}} should
  be removed from all URLs.
* Added FixPepperDocLinks() function to generate_docs.py. This will fix links
  for stable/beta/dev headers until this change makes it to stable.

BUG=none
R=sbc@chromium.org

Review URL: https://codereview.chromium.org/132283007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@247482 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 247482