Code reference should have the handbook sidebar.

[StevenDufresne]

Related to #2.

This makes the argument that the Code Reference should have the same sidebar as the handbooks.

Why

• The code already exists
• We want to create a consistent wp.org experience
• We can improve the handbooks at the same time.

Here's how it could look like:

Potential Improvements:

There are tickets in the meta repo that speak to this, although I don't have them in hand.

1. Use more of the page.
2. Fix the way items are visually nested
3. Smarter logic to expand the active section (to expand you have to click the arrow down, which is small,... clicking the title does something different)
4. Related: Make the search results appear in-line