Use hard links during Sphinx documentation tests

Hard linking the large files in ci/ should be faster than copying them
each time.  Also, let’s only remove them once; the “cleanup” routine was
duplicating work also done at the bottom of the file.

test-docs.sh

@@ -4,16 +4,12 @@ set -e
 cd "$(dirname ${BASH_SOURCE[0]})"
 
 # Copy in files that would otherwise need to download.
-cp ci/* skyfield/documentation
+ln -f ci/*.* skyfield/documentation
 
 function cleanup {
     cd skyfield/documentation
-    rm -f $(cd ../../ci; ls)
+    rm -f $(cd ../../ci; ls *.*)
 }
 trap cleanup EXIT
 
 make -C skyfield/documentation doctest
-
-# Remove the files we copied in.
-FILES="$(cd ci; ls *.*)"
-(cd skyfield/documentation; rm $FILES)