minor: moving introduction from the sidebar
DOCS-1790 Enterprise specific instructions with new yum and apt repos

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1790 Add Ubuntu and rhel/centos Enterprise instructions to TOC

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-1790: edits for new enterprise tutorials