fixed the home page Footer mobile responsive issue

[ShravaniAK]

currently, the Footer is not responsive,

I have made the Footer responsive on all screens,

[knative-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: ShravaniAK
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[linux-foundation-easycla]

• ✅login: ShravaniAK / (680801c)

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[knative-prow]

Welcome @ShravaniAK! It looks like this is your first PR to knative/docs 🎉

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[knative-prow-robot]

PR needs rebase.

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