Document `onDebugDynamicConfigurations:<debug type>` activation event and its purpose

[gjsjohnmurray]

The onDebugDynamicConfigurations activation event arrived in 1.45 but isn't yet listed here:

https://github.com/microsoft/vscode-docs/blob/bc2906544cb572b4b0f642b80b1bea0246a69870/api/references/activation-events.md?plain=1#L16-L18

The extension development doc doesn't yet mention the dynamic configurations scenario for which this activation event was introduced. It only covers dynamic provision of initial contents for a fresh launch.json, and for that the relevant activation event (already listed) is onDebugInitialConfigurations.

The vscode-mock-debug example extension demonstrates the use of onDebugDynamicConfigurations:<debug type> but its absence from documentation apart from the 1.45 release notes is probably limiting its uptake by extension developers.