update ignore file

.gitignore

-_old_/
-git_beginner/_build/doctrees/
-git_beginner/_build/html/
-git_beginner/_build/latex/
-git_beginner/_build/linkcheck/
-git_beginner/_build/singlehtml/
-linux_beginner/_build/certificates/
-linux_beginner/_build/doctrees/
-linux_beginner/_build/html/
-linux_beginner/_build/latex/
-linux_beginner/_build/singlehtml/
+_old_/*
+*/_build/*
+!*/_build/*.pdf
 linux_beginner/_static/1280px-Unix_history-simple.svg.png
 linux_beginner/_static/File-permissions-in-Linux-2.png
 linux_beginner/_static/Unix_history-simple.svg
-linux_beginner/clmystery-master
-linux_beginner/exercises_all
-linux_combined/_build/certificates/
-linux_combined/_build/doctrees/
-linux_combined/_build/html/
-linux_combined/_build/latex/
-linux_intermediate/_build/certificates/
-linux_intermediate/_build/doctrees/
-linux_intermediate/_build/html/
-linux_intermediate/_build/latex/
-linux_intermediate/_build/linkcheck/
-linux_intermediate/_build/singlehtml/
-linux_intermediate/attendees/*.csv
-linux_intermediate/attendees/*.pdf
-git_beginner/_build/certificates
-general/Signature_Dinkel.png
-general/Signature_Hodges.png
+linux_beginner/clmystery-master*
+linux_beginner/exercises_all*
+linux_intermediate/attendees/*
+linux_intermediate/attendees/*
+git_beginner/_build/certificates*
+general/Signature*.png
+general/Signature*.png