Revert "[Geometry] Remove the StoreGeometryReference specializations for our geometries"

This reverts commit 9e34b076f1c1b6a336e427bc46dfbda3ae3fde11, as those
specializations are still needed for the 2.3 release series of DUNE.
[Documentation] Mention RELEASE_NOTES.md in README.md
[Documentation] Bump version to 2.3.0-rc1
Merge branch 'releases/2.3'

Synchronize with release branch. The revert of the geometry patch was
actually taken out of the merge, as it must only be reverted for the 2.3
release series.

* releases/2.3:
  [Documentation] Bump version to 2.3.0-rc1
  [Documentation] Mention RELEASE_NOTES.md in README.md
  Revert "[Geometry] Remove the StoreGeometryReference specializations for our geometries"

Conflicts:
	README.md
	RELEASE_NOTES.md
	dune.module