0024269: Content of occt documentation should be formated

0024269: Content of occt documentation should be formated

building subsection introduced; wok moved to dev guides section;
Requirements and Installation sections were interchanged;
some Unicode characters removed from .md files; \DeclareUnicodeCharacter{00A0}{ } instruction added into refman file
images insertion rolled back to dual html,latex insertion; mainpage now is processed (index.tex);
surplus part of overview has been removed
foundation_classes.md and technical_overview.md updated;
Reviewed step, tobj, xde and partly iges; Corrections in other guides.
Overview installation and requirements changes updated