major overhaul to intro and docs in general

modules/doc/intro-01.rst

modules/doc/intro-02.rst

modules/doc/intro-03.rst

modules/doc/intro.rst

modules/doc/sel.png

modules/gfx/doc/cpk.png

modules/gfx/doc/custom.png

modules/gfx/doc/entity.rst

modules/gfx/doc/gfx.rst

modules/gfx/doc/hsc.png

modules/gfx/doc/line_trace.png

modules/gfx/doc/simple.png

modules/gfx/doc/sline.png

modules/gfx/doc/trace.png

modules/gfx/doc/tube.png

modules/gfx/pymod/export_entity.cc

modules/gfx/pymod/export_render_options.cc

modules/gui/src/file_browser.cc

modules/index.rst

modules/mol/base/doc/query.rst