index: update the changelog

Summarize all changes since v2.2.0 into the changelog.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>

source/index.rst

@@ -104,6 +104,14 @@ Creative Commons, PO Box 1866, Mountain View, CA 94042, USA.
        - Links refresh and additions, typos and syntax fixes, cosmetic changes,
          formatting conventions, notes movements, chapters changes, glossary
          adjustments
+   * - TBD
+     - 2.3.0
+     - - Formalize the Boot Manager requirements
+       - Require authenticated FMP capsules for firmware update
+       - Deprecate spin tables for AArch64
+       - Remove ambiguities around the collation protocol
+       - Bump UEFI, PSCI, SMCCC, BBR and dt-schema references versions
+       - Move some footnotes around
 
 .. toctree::
    :numbered: