Use Doxygen tag @ref for cross-references in documentation

Currently in many places in OCCT documentation (user and dev guides, overview, etc.) HTML-style references (#...tag...) are used to refer to other sections of the documentation. This works, to some extent, in HTML, but generates non-working links in PDF. Instead, Doxygen tag @ref should be used for cross-references in documentation, like this:

@ref occt_coding_rules_3 "Formatting Rules"

instead of

#occt_coding_rules_3