DDoc: Doc improvements for std.algorithm (see extended description)

* Links to information about ranges (it is confusing for a new user to see ranges being mentioned everywhere but no description of what a range is). (the links are to std.range and a chapter of Ali Cehreli's D Programming Language Tutorial book)
* Example (as opposed to just mention) on using copy as std::backward_copy, and UCFS (where it is intuitive) in copy documentation.
* Moved all 'See also' links to See_Also: at the ends of documentation blokcs.