chore: import upstream snapshot with attribution

proc/help/nice.html

@@ -0,0 +1,10 @@
+<header>Nice Level</header>
+
+Every process runs with a certain priority, called the nice level. A high
+priority process will get more CPU time than those with lower priorities,
+and thus appear to run faster. Changing the nice level only really makes
+sense for CPU-intensive processes, as most programs spend almost all of
+their time waiting for something (such as user input) and thus will not
+benefit or suffer if their priority changes.
+<hr>
+