doc/articles: add {{donotedit}} to templates

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5502088

doc/articles/defer_panic_recover.html

 <!-- Defer, Panic, and Recover -->
+<!--
+  DO NOT EDIT: created by
+    tmpltohtml articles/defer_panic_recover.tmpl
+-->
 
 <p>
 Go has the usual mechanisms for control flow: if, for, switch, goto.  It also

doc/articles/defer_panic_recover.tmpl

 <!-- Defer, Panic, and Recover -->
-
+{{donotedit}}
 <p>
 Go has the usual mechanisms for control flow: if, for, switch, goto.  It also
 has the go statement to run code in a separate goroutine.  Here I'd like to

doc/articles/error_handling.html

 <!-- Error Handling and Go -->
+<!--
+  DO NOT EDIT: created by
+    tmpltohtml articles/error_handling.tmpl
+-->
 
 <p>
 If you have written any Go code you have probably encountered the built-in

doc/articles/error_handling.tmpl

 <!-- Error Handling and Go -->
-
+{{donotedit}}
 <p>
 If you have written any Go code you have probably encountered the built-in
 <code>error</code> type. Go code uses <code>error</code> values to