IDE.md: Add Eclipse 2023-12 update sites for AJDT

Signed-off-by: Alexander Kriegisch <Alexander@Kriegisch.name>

docs/developer/IDE.md

@@ -32,6 +32,13 @@ projects using AspectJ Maven Plugin.
 #### AspectJ Development Tools (AJDT)
 
 Use an update site corresponding to your Eclipse version (minimal version listed):
+* Eclipse 2023-12 (4.30), AspectJ 1.9.21, Java 21: https://download.eclipse.org/tools/ajdt/430/dev/update<br>
+  **Please note:** In Eclipse 2023-12, you should also add
+  https://download.eclipse.org/eclipse/updates/4.31-I-builds/I20231201-1800/ in addition to the regular release update
+  sites https://download.eclipse.org/eclipse/updates/4.30/ or https://download.eclipse.org/releases/latest. This is,
+  because AspectJ 1.9.21 contains support for JDK 21 string templates, which was added to the compiler after the 2023-12
+  release, but also includes a few Eclipse API changes requiring the additional update site. After adding it, please
+  make sure to call _Help_ → _Check for Updates_ and accept the updates.
 * Eclipse 2022-12 (4.26), AspectJ 1.9.19, Java 19: https://download.eclipse.org/tools/ajdt/426/dev/update
 * Eclipse 2022-03 (4.23), AspectJ 1.9.19, Java 19: https://download.eclipse.org/tools/ajdt/423/dev/update
 * Eclipse 2021-09 (4.21), AspectJ 1.9.8, Java 17: https://download.eclipse.org/tools/ajdt/421/dev/update
@@ -49,6 +56,10 @@ update sites:
   * Eclipse 2020-12 (4.18): https://repo.t5.fi/public/eclipse/m2eclipse-ajdt/eclipse-2020-12/
   * Eclipse 2019-06 (4.12): https://repo.t5.fi/public/eclipse/m2eclipse-ajdt/eclipse-2019-06/
 
+Your AspectJ-related list of Eclipse update sites could look like this for Eclipse 2023-12:
+
+![Eclipse IDE filtered list of update sites](EclipseSoftwareSitesAJDT.png)
+
 ### IntelliJ IDEA
 
 IDEA is a commercial IDE by JetBrains, which also has a free Community Edition. The author of this guide uses a