.gitattributes: Hide Documentation/_static.

Treat Documentation/_static and SVG files as binary in git so that when
performing a git diff or git grep, it prints just one line diff rather
than matching the content of some terse javascript file. This reduces
the noise greatly for greps of short strings to instead print the
following:

    $ git grep "\.fo"
    Binary file Documentation/_static/copybutton.js matches

Signed-off-by: Joe Stringer <joe@cilium.io>

.gitattributes

@@ -3,3 +3,5 @@ examples/kubernetes/connectivity-check/connectivity-*.yaml linguist-generated
 pkg/k8s/apis/cilium.io/v2/client/crds/*.yaml linguist-generated
 Documentation/cmdref/** linguist-generated
 Documentation/helm-values.rst linguist-generated
+Documentation/_static/* -diff
+*svg -diff