Add information about v15 and PHP8.2

[mikelittle]

This addresses issue https://github.com/humanmade/altis-documentation/issues/475

[missjwo]

Note: 8.2 is marked as beta support in WordPress which is why its set as experimental here.

https://make.wordpress.org/core/handbook/references/php-compatibility-and-wordpress-versions/

Props to @mikelittle for pointing this out to me.