runtime(doc): symlinking netrw.txt causes problems during install on Windows

So let's remove the symlink and copy the netrw documentation back into
runtime/doc directory. While at it, add a Makefile target to do this
whenever runtime/pack/dist/opt/netrw/doc/netrw.txt is updated.

fixes: #16878
fixes: #16872

Co-authored-by: Brandon Maier <brandon.maier@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/Makefile b/runtime/doc/Makefile
index 82b5680..e278933 100644
--- a/runtime/doc/Makefile
+++ b/runtime/doc/Makefile
@@ -141,6 +141,9 @@
 os_win32.txt:
 	touch $@
 
+pi_netrw.txt: ../pack/dist/opt/netrw/doc/netrw.txt
+	cp ../pack/dist/opt/netrw/doc/netrw.txt $@
+
 vietnamese.txt:
 	touch $@