Add release date for 0.20.0

[ndgrigorian]

This PR adds the release date of 0.20.0 to the changelog

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[github-actions]

Deleted rendered PR docs from intelpython.github.com/dpctl, latest should be updated shortly. 🤞

[github-actions]

Array API standard conformance tests for dpctl=0.20.0rc2=py310h93fe807_10 ran successfully.
Passed: 1116
Failed: 5
Skipped: 119

[coveralls]

coverage: 84.978%. first build
when pulling 95b7c35 on add-release-date-for-0.20.0
into fecef55 on master.