Updated source, .gitignore, added texdoc

  * source/latex/isuth-modules/isuth-modules.org: now it is source for
  * all DCF files in tex/latex/isuth-modules, they are tangled from this
  * Org document;
  * texdoc/texdoc.cnf: new file, explicitly set okular as preferred PDF viewer;
  * .gitignore: ignore HTML files.

Signed-off-by: Vladimir Lomov <lomov.vl@gmail.com>