0031635: Documentation - minor clean-up of User Guide for Foundation Classes module

0031635: Documentation - minor clean-up of User Guide for Foundation Classes module

Changes in Technical Overview:
Corrected reference of "Mesh" as dedicated module, which is actually a Toolkit within "Modeling Algorithms" module.
Reduced references to "Configurable optimized memory manager",
which has not been updated for a long time and no more encouraged to be used as replacement of standard memory allocator of C library.
VRML/STL converters have been moved from section Mesh to Data Exchange.
Visualization chapter has been slightly reworded and reordered;
added references to PBR metallic-roughness material model.
Data Exchange chapter - reworded sentence "The exchanges run smoothly regardless of the quality of external data..."
to "This module handles various problems of interoperability between CAD systems...";
added AP242 to the list of STEP capabilities.

Changes in User Guide for Foundation Classes:
Removed references to "physical quantities" in package Quanity, deprecated since #0028799 and not actually used anywhere.
Adjusted description of string classes to avoid duplicated statements.
Collections section has been refactored to replace obsolete TCollection references by NCollection.
Remove obsolete statement "furthermore *catch()* statement does not allow passing exception object as argument".

Fixed broken nested enumeration in a couple of places.
Fixed usage of inappropriate quote symbols in code snippets.
Fixed references to non-existing classes like ZeroDivide instead of Standard_DivideByZero.
Removed suggested usage of Standard_Failure::Raise() instead of "throw Standard_Failure()" following #0026937.
Code snippets now suggest "catch(const Standard_Failure& )" instead of "catch(Standard_Failure )" (#0026937).
Fixed broken code snippets with lost symbols like <<.

Cosmetics: removed numerous trailing spaces, double spaces.
Added missing {.cpp} language qualifier to code quotations.
Sentences within paragraphs have been split into multiple lines in source file.