Documentation: update .gitignore files

Add some missing files to .gitignore.
Push Documentation/.gitignore down into subdirectories.

Signed-off-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/Documentation/timers/.gitignore b/Documentation/timers/.gitignore
new file mode 100644
index 0000000..c5c45d7
--- /dev/null
+++ b/Documentation/timers/.gitignore
@@ -0,0 +1 @@
+hpet_example