Amend changelog with introductory information

CHANGELOG.rst

@@ -9,6 +9,16 @@ and this project adheres to `Semantic Versioning`_.
 `3.0b0` - 2023-02-15
 --------------------
 
+This release starts a new era of the AlekSIS® framework, by introducing a
+dynamic frontend app written in Vue.js which communicates with the backend
+through GraphQL.  Support for legacy views (Django templates and
+Materialize) was removed; while there is backwards compatibility for now,
+this is only used by official apps until their views are fully migrated.
+
+AlekSIS and its new frontend require Node.js version 18 or higher to run the
+Vite bundler. On Debian, this means that Debian 12 (bookworm) is needed, or
+Node.js must be installed from a third-party repository.
+
 Removed
 ~~~~~~~