Bug 1608201 - Links broken in contributing docs.r=sylvestre

Differential Revision: https://phabricator.services.mozilla.com/D59621

--HG--
extra : moz-landing-system : lando

docs/config.yml

@@ -55,3 +55,5 @@ redirects:
     toolkit/components/telemetry/telemetry: toolkit/components/telemetry
     tools/compare-locales/index.html: build/buildsystem/locales.html
     tools/docs/index.html: tools/moztreedocs/index.html
+    tools/docs/contribute/how_to_contribute_firefox.html: contributing/how_to_contribute_firefox.html
+    tools/docs/contribute/directory_structure.html: contributing/directory_structure.html