Move handbook font to system default

[CoreyBenefiel]

Description: As part of the rebranding activities we have updated the Company handbook and other sites like H2. Due to some rendering issue we moved H2 font to WordPress Core default font stack. See ticket here https://github.com/humanmade/h2/pull/569.

We would like to try the same approach for the Handbook to bring some styling consistency across our tools.

Acceptance Criteria:

• The default font stack approach is implemented in the Handbook with no adverse impact.

[kadamwhite]

@CoreyBenefiel This is ready for review now https://handbook.hmn-dev.altis.cloud/

[CoreyBenefiel]

This looks good, thanks @kadamwhite.This can go to prod when you have a chance, please.