FEATURE: Add dropdown navigation bar

[awmulyadi]

Is your feature request related to a problem? Please describe.
The current form of navigation bar doesn't allow for multiple sub-pages under the main categories. Since a literature survey could have a tree-like categorization with multiple sub-categories, the website needs to consider this aspect.
As an example, please see the coarse-grained causal reasoning in the prior work investigation.

Describe the solution you'd like
A dropdown navigation bar in the header would be an ideal feature to facilitate this needs.

Describe alternatives you've considered
n/a

Additional context (optional)
n/a