Update h1,h2,h3 font sizes in the docs pages (#2938)

* Update h1,h2,h3 font sizes

Signed-off-by: Tynan DeBold <thdebold@gmail.com>

* Update h2 font size

Signed-off-by: Tynan DeBold <thdebold@gmail.com>

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Signed-off-by: Tynan DeBold <thdebold@gmail.com>
Co-authored-by: Jo Stichbury <jo_stichbury@mckinsey.com>
Co-authored-by: Juan Luis Cano Rodríguez <juan_luis_cano@mckinsey.com>