Improve navigation between sections

[iammasterbrucewayne]

Problem: Navigation between sections has a lot friction if someone wants to move quickly between lessons or skip some parts. Since the navigation uses a drawer, it forces the users to always EITHER use the lesson interface OR the sidebar navigation.

Proposed Solution:

• [x] A sidebar similar to mdbook format where all sections and sub-sections (collapsible) are displayed together.
• [ ] It should be possible for the user to continue using the lesson interface with the sidebar open - again, similar to mdbook format.
• [ ] Allow hotkeys for navigation (good to have)

[iammasterbrucewayne]

https://github.com/dotcodeschool/frontend/commit/44305a012308e36bf9b31f5adf961eac5e19c90c fixes the primary concern

keeping the navbar open and using hotkeys are low impact, hence not a priority