docs: remove postcss from install script

[ingvoo]

Remove postcss as a getting started requirement

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Remove unneeded postcss dependency to be installed to get started as documented on the getting started docs page

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[farnabaz]

Thanks