remove and stop tracking built cheatsheets and handouts

- remove out-of-date artifacts
- add build artifacts to gitignore
- update README to link up-to-date artifacts at matplotlib.org

Author: jimustafa

.gitignore

@@ -1,3 +1,8 @@
+# built cheatsheets and handouts
+# ----------------------------------
+cheatsheets*.p[dn][fg]
+handout-*.p[dn][fg]
+
 # TeX auxiliary files
 # ----------------------------------
 *.aux

README.md

@@ -1,16 +1,16 @@
 # Cheatsheets
 
-![](./cheatsheets-1.png)
+![](https://matplotlib.org/cheatsheets/cheatsheets-1.png)
 
-![](./cheatsheets-2.png)
+![](https://matplotlib.org/cheatsheets/cheatsheets-2.png)
 
 # Handouts
 
-![](./handout-beginner.png)
+![](https://matplotlib.org/cheatsheets/handout-beginner.png)
 
-![](./handout-intermediate.png)
+![](https://matplotlib.org/cheatsheets/handout-intermediate.png)
 
-![](./handout-tips.png)
+![](https://matplotlib.org/cheatsheets/handout-tips.png)
 
 # How to compile