Add highlights from 0.20.0 to changelog

[ndgrigorian]

This PR adds a short summary of highlighted features from the 0.20.0 release

• Have you provided a meaningful PR description?
• Have you added a test, reproducer or referred to an issue with a reproducer?
• Have you tested your changes locally for CPU and GPU devices?
• Have you made sure that new changes do not introduce compiler warnings?
• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?
• If this PR is a work in progress, are you opening the PR as a draft?

[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_12 ran successfully.
Passed: 1114
Failed: 7
Skipped: 119

[coveralls]

coverage: 84.972%. remained the same
when pulling 1e344b1 on add-highlights-from-0.20.0
into 4e2fa86 on master.