Add release notes for 0.20.2 (pydata#6060)

whats-new for 0.20.1 (pydata#5943)

v0.20 Release notes (pydata#5924)

* release notes

* fix version number

* Update doc/whats-new.rst

Co-authored-by: Stephan Hoyer <shoyer@google.com>

* fix length of title underline

Co-authored-by: Stephan Hoyer <shoyer@google.com>

v0.19.0 release notes (pydata#5632)

* Revert "Use _unstack_once for valid dask and sparse versions (pydata#5315)"

This reverts commit 9165c26.

* 0.18.2 release notes

* fix RTD [skip-ci] (pydata#5518)

* v0.19.0 release notes

* Update doc/whats-new.rst [skip-ci]

* remove empty sections

Co-authored-by: Maximilian Roos <m@maxroos.com>
Co-authored-by: keewis <keewis@users.noreply.github.com>

0.18.2 release notes

0.18.1 release notes (pydata#5340)

* 0.18.1 release notes

Release-workflow: Bug fix (pydata#5273)