Overview

The goal of this pr is do some cleanup related to the lesson pages.

Specifically, this includes:

Rendering markdown inside excerpts
Updating logic for "next" and "previous" lesson links to:
- Skip over "future" (or unfinished) lessons
- Transition users to next section of lessons if they press "next" from the last lesson in a section or "previous" in the first lesson in a section
Removing empty excerpts from the top of the lesson page
Restyling the "next" and "previous" links at the bottom of the lesson page to align better with the "next" and "previous" links
Adding Benchee lesson to the menu