Add stub directory-level documentation for most source directories

This includes app, core, feature, lib, and tools, but excludes
ext, test, and trunnel.

This was generated by the following shell script:

cd src
for dname in $(find lib core feature app tools -type d |grep -v \\.deps$); do
    keyword="$(echo "$dname" |sed -e "s/\//_/" )"
    target="${dname}/${keyword}.dox"
    echo "$target"
    cat <<EOF >"$target"
/**
@dir ${dname}
@brief ${dname}
**/
EOF

    git add "$target"
done

src/app/app.dox

+/**
+@dir app
+@brief app
+**/

src/app/config/app_config.dox

+/**
+@dir app/config
+@brief app/config
+**/

src/app/main/app_main.dox

+/**
+@dir app/main
+@brief app/main
+**/

src/core/core.dox

+/**
+@dir core
+@brief core
+**/

src/core/crypto/core_crypto.dox

+/**
+@dir core/crypto
+@brief core/crypto
+**/

src/core/mainloop/core_mainloop.dox

+/**
+@dir core/mainloop
+@brief core/mainloop
+**/

src/core/or/core_or.dox

+/**
+@dir core/or
+@brief core/or
+**/

src/core/proto/core_proto.dox

+/**
+@dir core/proto
+@brief core/proto
+**/

src/feature/api/feature_api.dox

+/**
+@dir feature/api
+@brief feature/api
+**/

src/feature/client/feature_client.dox

+/**
+@dir feature/client
+@brief feature/client
+**/

src/feature/control/feature_control.dox

+/**
+@dir feature/control
+@brief feature/control
+**/

src/feature/dirauth/feature_dirauth.dox

+/**
+@dir feature/dirauth
+@brief feature/dirauth
+**/

src/feature/dircache/feature_dircache.dox

+/**
+@dir feature/dircache
+@brief feature/dircache
+**/

src/feature/dirclient/feature_dirclient.dox

+/**
+@dir feature/dirclient
+@brief feature/dirclient
+**/

src/feature/dircommon/feature_dircommon.dox

+/**
+@dir feature/dircommon
+@brief feature/dircommon
+**/

src/feature/dirparse/feature_dirparse.dox

+/**
+@dir feature/dirparse
+@brief feature/dirparse
+**/

src/feature/feature.dox

+/**
+@dir feature
+@brief feature
+**/

src/feature/hibernate/feature_hibernate.dox

+/**
+@dir feature/hibernate
+@brief feature/hibernate
+**/

src/feature/hs/feature_hs.dox

+/**
+@dir feature/hs
+@brief feature/hs
+**/

src/feature/hs_common/feature_hs_common.dox

+/**
+@dir feature/hs_common
+@brief feature/hs_common
+**/