Add Landing Page Project Roadmap to docs

[joshenx]

Description

This PR adds the roadmap to revamp the landing page as per https://github.com/WING-NUS/SSID/issues/317

Related Issue

https://github.com/WING-NUS/SSID/issues/317

Motivation and Context

Current landing page of SSID is outdated + basic

How Has This Been Tested?

Reviewed documentation

Screenshots (if appropriate):

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [X] My code follows the code style of this project.
• [X] My change requires a change to the documentation.
• [X] I have updated the documentation accordingly.

[huyuxin0429]

Please place the roadmap in slack, and modify the title and contents to reflect changes made.

[joshenx]

@huyuxin0429 I have modified the contents to reflect the changes made (Add Landing Page Project Roadmap to docs)