fix headers and spacing on /about

[AlexJBanks]

<h4>s need to be converted to <h3>s and main.scss needs adjusting to give better options for heading sizes

[diegoherrer4]

I can help

[MattyTheHacker]

I can help

Feel free to open a PR!

[hugobiais]

Should we convert all <h4>s to <h3>s (including the ones in the buttons) or just the ones under the "Get involved" section ?
And what do you mean by "give a better option for heading sizes" ? Could you be more precise on that ? Do you mean adjusting the font-size ?