Fix heading hierarchy and spacing rhythm in Get Involved section

[AbhinavJadhav001]

Bring the get-involved card's heading levels (h4/h5) up to match the h3+p rhythm used in the project cards (project_list.html / featured-projects.html), and add margin-bottom-big spacing between link items for consistent vertical rhythm.

Fixes #947

Fixes

• Fixes #[issue number] by @[issue author]

Description

Fixes the typography size differentiation and vertical rhythm issue in the
"Get Involved" homepage section.

Technical details

Tests

Screenshots

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