Clean up intro; standardize import commands on the Std Lib index page.

--Cleaned up the introductory paragraph on this page.

--Implemented consistent styles for adding the Std Lib to a user's policy:

"To import elements of the CFEngine Standard Library into your CFEngine policy, enter the following:"

"To import the entire CFEngine Standard Library, enter the following:"

"To use these bundles, add the following to your policy:"