cleanup: insert .idea/ in file .gitignore (#6065)
cleanup: insert .idea/ in file .gitignore (#6065)

Ignore folder .idea/ when open project Envoy with
IntelliJ IDEA.

Signed-off-by: Nguyen Hai Truong <79e9aa0c2e274ca51ed3c3b9112f68929f539784@vn.fujitsu.com>
dnoe/envoy,istio/envoy,dnoe/envoy,envoyproxy/envoy,dnoe/envoy,lyft/envoy,eklitzke/envoy,istio/envoy,envoyproxy/envoy-wasm,eklitzke/envoy,envoyproxy/envoy-wasm,dnoe/envoy,istio/envoy,eklitzke/envoy,lizan/envoy,eklitzke/envoy,lizan/envoy,jrajahalme/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy,lizan/envoy,jrajahalme/envoy,jrajahalme/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,eklitzke/envoy,lyft/envoy,lizan/envoy,jrajahalme/envoy,lyft/envoy,lizan/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,istio/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,eklitzke/envoy,istio/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,dnoe/envoy,envoyproxy/envoy,jrajahalme/envoy,lyft/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy