[NaCl SDK Docs] Fix README, cleanup Makefile, remove devsite stuff.

BUG=341480
R=jfb@chromium.org, sbc@chromium.org

Review URL: https://codereview.chromium.org/180003003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253268 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253268