Add changelog for 0.21.0 release

[ndgrigorian]

This PR updates the changelog for the 0.21.0 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=0.21.0=py310h93fe807_21 ran successfully.
Passed: 1115
Failed: 6
Skipped: 119

[coveralls]

coverage: 85.264%. remained the same
when pulling 2c6633e on add-changelog-for-0.21.0
into bf68438 on master.

[antonwolfy]

LGTM!