Improve Typography Design

[mildronize]

Problem

1. Less y-axis margin between code block and bullet

2. More y-axis margin between bullet item and its nested item.

3. On mobile, font ratio between title and heading is not properly.

4. On mobile, the URL don't be wrapped to new line

5. Font weight between title and heading is not properly.

[aaronamm]

@mildronize could you please open a PR to fix this issue? Thank you so much.