v0.19.0 release notes (#5632)

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

This reverts commit 9165c26.

* 0.18.2 release notes

* fix RTD [skip-ci] (#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>

doc/whats-new.rst

@@ -14,10 +14,23 @@ What's New
 
     np.random.seed(123456)
 
-.. _whats-new.0.18.3:
 
-v0.18.3 (unreleased)
----------------------
+.. _whats-new.0.19.0:
+
+v0.19.0 (23 July 2021)
+----------------------
+
+This release brings improvements to plotting of categorical data, the ability to specify how attributes
+are combined in xarray operations, a new high-level :py:func:`unify_chunks` function, as well as various
+deprecations, bug fixes, and minor improvements.
+
+
+Many thanks to the 29 contributors to this release!:
+
+Andrew Williams, Augustus, Aureliana Barghini, Benoit Bovy, crusaderky, Deepak Cherian, ellesmith88,
+Elliott Sales de Andrade, Giacomo Caria, github-actions[bot], Illviljan, Joeperdefloep, joooeey, Julia Kent,
+Julius Busecke, keewis, Mathias Hauser, Matthias Göbel, Mattia Almansi, Maximilian Roos, Peter Andreas Entschev,
+Ray Bell, Sander, Santiago Soler, Sebastian, Spencer Clark, Stephan Hoyer, Thomas Hirtz, Thomas Nicholas.
 
 New Features
 ~~~~~~~~~~~~
@@ -105,10 +118,6 @@ Bug fixes
   By `Augustus Ijams <https://github.com/aijams>`_.
 
 
-Documentation
-~~~~~~~~~~~~~
-
-
 Internal Changes
 ~~~~~~~~~~~~~~~~
 - Run CI on the first & last python versions supported only; currently 3.7 & 3.9.
@@ -117,7 +126,6 @@ Internal Changes
 - Publish test results & timings on each PR.
   (:pull:`5537`)
   By `Maximilian Roos <https://github.com/max-sixty>`_.
-
 - Explicit indexes refactor: add a ``xarray.Index.query()`` method in which
   one may eventually provide a custom implementation of label-based data
   selection (not ready yet for public use). Also refactor the internal,
@@ -162,13 +170,6 @@ New Features
 - Raise more informative error when decoding time variables with invalid reference dates.
   (:issue:`5199`, :pull:`5288`). By `Giacomo Caria <https://github.com/gcaria>`_.
 
-Breaking changes
-~~~~~~~~~~~~~~~~
-
-
-Deprecations
-~~~~~~~~~~~~
-
 
 Bug fixes
 ~~~~~~~~~