feat(design): add documentation and community sections

[rexx010]

Summary

This PR introduces the UI designs for the documentation and community-focused sections of the landing page, providing developers with a clear onboarding experience, navigation, project roadmap, and contribution resources.

Implemented

• Designed the Installation Docs page.
• Designed the Documentation Navigation section.
• Designed the Roadmap section.
• Designed the Community & Contribution section.

Goals

• Improve the developer onboarding experience.
• Make documentation easier to navigate and discover.
• Clearly communicate the project's roadmap and future direction.
• Encourage community participation and open-source contributions.
• Maintain a responsive and consistent design aligned with the project's design system.

Type of Change

• UI/UX Design
• Bug Fix
• New Feature
• Breaking Change
• Documentation Update

Notes

These changes are design-focused and introduce no functional or backend logic changes.

Figma Link

https://www.figma.com/design/BSr242a615D8Lbg6WjT7l1/Use-stellar-design?node-id=0-1&p=f&t=ev4q750WePag3udj-0

Closes #125
Closes #126
Closes #127
Closes #129

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