Improve gitignore. about b2

[mloskot]

Candidates to ignore inside the example/:

$ cd example
$ for d in `find -type d`; do pushd $d && ~/b2/b2 ; popd ; done
...
$ git status
On branch main
Your branch is up to date with 'origin/main'.

Untracked files:
  (use "git add <file>..." to include in what will be committed)
        asciidoctor/bin/
        built_tool/bin/
        built_tool/core/bin/
        built_tool/tblgen/bin/
        complex-testing/bin/
        customization/bin/
        generate/bin/
        generator/bin/
        gettext/bin/
        hello/bin/
        libraries/app/bin/
        libraries/bin/
        libraries/util/foo/bin/
        make/bin/
        named-install-dirs/bar/
        named-install-dirs/foo/
        named-install-dirs/q/
        pch-multi/bin/
        pch/bin/
        pkg-config/bin/
        qt/qt4/hello/bin/
        qt/qt4/moccable-cpp/bin/
        qt/qt4/uic/bin/
        sanitizers/bin/
        sass/bin/
        testing/bin/
        time/bin/
        try_compile/bin/
        variant/bin/
        variant/libs/bin/