Add explanation/concept for extension maturity model #43713

Job	Run time
CI Sanity Check	32s
Attach pull request number	2s
Initial JDK 17 Build	11m 32s
Calculate Test Jobs	33s
JVM Tests - JDK 17	2m 41s
JVM Tests - JDK 21	3m 19s
Gradle Tests - JDK ${{ matrix.java.name }}	0s
Maven Tests - JDK ${{ matrix.java.name }}	0s
JVM Tests - JDK 17 Windows	4m 14s
Devtools Tests - JDK ${{ matrix.java.name }}	0s
Kubernetes Tests - JDK ${{ matrix.java.name }}	0s
Quickstarts Compilation - JDK 17	6m 21s
Native Tests - ${{ matrix.category }}	0s
MicroProfile TCKs Tests	0s
Native Tests - Virtual Thread - ${{ matrix.category }}	0s
Upload build stats to collector	0s
Build report	34s
	29m 48s

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