Fix VERSION in Makefile and update documentation for string_view_t

[dantargz]

Add information to the docs about string_view_t and the overloads that use it.

Document generation was accidentally broken in previous commit from adding the word "version"

% make docs/manual.html   
asciidoctor -b html5 -a version=1.14 of pugixml this macro will become obsolete. docs/manual.adoc -o docs/manual.html
asciidoctor: FAILED: input file of is missing
make: *** [docs/manual.html] Error 1

The result of the old sed command:

% sed -n 's/.*version \(.*\).*/\1/p' src/pugiconfig.hpp 
1.14
of pugixml this macro will become obsolete.

Tweaked the sed command to /q quit after the first occurance of the word "version" -- my sed knowledge is weak, there may be a better way to do this.