Currently, if you're viewing a how-to guide and you click "How-to
Guides" in the sidebar, you aren't navigated back to the index page (it
will work if you click on a different guides section). To get back to
the index page, you need to scroll up and click the breadcrumbs.
After this change, clicking the link in the sidebar should navigate you
to the index page regardless of the page you are viewing.
Only side-effect from what I can tell is that "Home > Introduction" just
becomes "**Home**", which I think is fine (maybe preferable).
Before:

After:

Some docs layout improvements to help guide user journey.
Currently we have `Home | Tutorials | How-tos | Concepts | Reference` in
top-level horizontal navigation bar.
Here we make these updates:
- Top-level horizontal navigation bar is just `Home | API Reference`
- Add vertical sidebar to `Home` with sections:
- Introduction
- Get started
- Guides
- Resources
`Get Started` contains quickstarts for LG and LG Platform / deployment.
These are tutorials in Diataxis terms.
`Guides` contains index pages for how-tos, concepts, tutorials.
Advantage of this organization is that users are directed naturally down
the sidebar from Intro -> Get started -> How-tos, which is roughly how
we expect them to proceed.
This also makes deployment info more accessible as it is highlighted in
the "Getting started" section.
