refactor: convert config symlink to empty file for Forestry

Otherwise, Windows 10 users must either enable Developer mode or run
git clone with admin privileges as well as using the special
`git clone -c core.symlinks=true https://...` command.

config.toml

@@ -0,0 +1,7 @@
+# Academic Website Builder
+# https://sourcethemes.com/academic/
+#
+# The presence of this file is for compatibility with Blogdown and Forestry.
+# If you are not using those tools, this file may be deleted.
+#
+# The actual configuration files are stored in the `config/_default/` folder.