[Sidebar] Update UI/UX discrepancy

[th-94]

Prerequisites

• I have searched for duplicate or closed issues
• I have read the contributing guidelines

Describe the issue

1. The section label of the sidebar is currently focusable with a pointer cursor which could mislead that it is clickable but it actually isn't when collapsible prop is not set.
2. Adding space to the content section and bottom section according to Figma design.

What operating system(s) are you seeing the problem on?

macOS

What browser(s) are you seeing the problem on?

No response

Describe your frontend stack. What version of React and @govtechsg/sgds-web-component are you using? CSR or SSR?

3.24.0