Change doc/.gitignore to make git aware of files with non-generated contents.

(cherry picked from commit 1bf9e124eb)

doc/.gitignore

@@ -1,3 +1,12 @@
-# most things here are generated, so ignore everything, and explicitly
-# add things that need to be added -- use `git add -f' in such cases.
-*
+# ignore all sub-directories and generated files
+/*/
+/*.html
+/*.css
+/*.js
+/*.sxref
+/blueboxes.rktd
+/docindex.sqlite
+
+# except directories that have committed contents
+!/release-notes/
+!/*-std/