Docs: add navbar.md for mojo docs

No change to logic, only docs. This adds navbar.md (supported by gitiles
markdown) to the mojo docs. This makes http://go/mojo-docs/ slightly
easier to navigate, as the top bar will link to some of the high-level
documents. This does not link to every mojo doc, to avoid
over-cluttering the navbar.

Test: Upload to gerrit > open file > click "gitiles"
Change-Id: I2cad169397ae8a0d12b23da735fb9d641a1eff64
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824062
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699991}

mojo/navbar.md

+# Mojo docs (go/mojo-docs)
+
+* [Home](/mojo/README.md)
+* [Intro to Mojo & Services](/docs/mojo_and_services.md)
+* [Mojo Basics](/mojo/docs/basics.md)
+* [IDL](/mojo/public/tools/bindings/README.md)
+* [C++ bindings](/mojo/public/cpp/bindings/README.md)
+* [Chromium's Mojo style guide](/docs/security/mojo.md)
+
+[home]: /mojo/README.md