docs: Add link in navbar to file a bug or report security issue
BRANCH=none
TEST=View in gitiles
* [Home][home]
* [Documentation](/docs/sitemap.md)
* [Getting Started Quickly](/docs/getting_started_quickly.md)
+* [File a Bug](https://bugs.chromium.org/p/chromium/issues/entry?components=OS%3EFirmware%3EEC)
+* [Report Security Issue](https://chromium.googlesource.com/chromiumos/docs/+/master/reporting_bugs.md#security)
* [Chromium OS Docs](https://chromium.googlesource.com/chromiumos/docs/+/master/README.md)
