Improve the side menu when both the content and the design are ready

[cindyli]

Is your feature request related to a problem? Please describe.

The side menu is initially designed to display secondary navigation items (h2s) to allow the jump to each section in the article. The style of it has been implemented based on the mock data. Improving it to a fully functioning feature is waiting on 2 materials:

1. Contents with h2s. When contents are available, they need to be parsed to extract h2s for display;
2. "News" and "Views" pages will not have h2s but only have the title in h1. @lliskovoi will come up with a design for these pages.

Describe the solution you'd like

See above.

Describe alternatives you've considered

N/A.

Additional context

N/A.