Add 4.6.1 release notes.

Author: thijskh

docs/release_notes/4.6.1.md

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+# OpenConext EngineBlock v4.6.1 Release Notes #
+
+Improvements to new Material UI.
+* On the WAYF, search in IdP keywords in a case-insensitive manner.
+* Open TOS in new browser window as to not disturb WAYF.
+* Remove not so useful 'go back' from waiting page.
+* Ensure clicking "Press enter to select" location forwards to IdP.
+* Remove old-style logging in Material theme footer.
+* When removing a previous-choice IdP, move it to the regular IdP list.
+* Center consent approve and reject buttons on mobile devices.
+* Remove excessive DOM reading and writing during every mouse move.