Bug 1539128 - Add fluent docs to the docs sidebar under Localization. r=flod

Differential Revision: https://phabricator.services.mozilla.com/D24879

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

intl/docs/index.rst

@@ -24,3 +24,4 @@ use Mozilla's I18n APIs.
    locale
    dataintl
    localization
+   l10n/l10n/index