Extension API Publish: Fix package version and improve developer documentation

Follow-up actions now that we published the first extension API package here: https://github.com/eclipse-cdt-cloud/vscode-peripheral-inspector/pkgs/npm/vscode-peripheral-inspector

* The idea was that CI determines the extension version and applies it for the npm package publish. But this failed. The first published version is `0.0.0` while `1.9.2` was expected. This needs to be fixed before the next real changes are introduced to the API and would override older versions in the registry.
* We need to update https://github.com/eclipse-cdt-cloud/vscode-peripheral-inspector/blob/main/docs/extending-peripheral-inspector.md with instructions how to properly pull the package. In particular how to set up the required npm registry (for example add this in an `.npmrc` file: `@eclipse-cdt-cloud:registry=https://npm.pkg.github.com`
* We should consider if we can/should change the README shown in the package registry with something more developer focused like the above "extending...." manual.