Styles: Sidebar style refresh

[danielledeleo]

Proposed changes

Implements a refreshed sidebar with a slightly smaller font, less clutter, a little more padding, text contrast changes, and a new Overview link visible on sections with a custom landing page.
### Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and CHANGELOG.md)

[danielledeleo]

TODO:

• Add dropdown arrows
• Top level page links don't get highlighted