Generate list of ML modules in readmes

Per discussion in https://github.com/elastic/integrations/pull/2640#issuecomment-1029550325, there are several packages that already include machine learning modules, but the details about those modules are not mentioned in the integration readmes (and therefore not in the integration documentation).

We've drafted some examples of what that content would ideally look like in these PRs:
* https://github.com/elastic/integrations/pull/2640
* https://github.com/elastic/integrations/pull/2662
* https://github.com/elastic/integrations/pull/2352
* https://github.com/elastic/integrations/pull/2115

However, given that those tables contain strings that are copied and pasted from the ML module manifests (e.g. https://github.com/elastic/integrations/blob/main/packages/nginx/kibana/ml_module/nginx-Logs-ml.json), they would be easier to maintain (and less likely to get out-dated) if they were automatically generated as part of the package build (i.e. similar to how the logs and metrics details are added to the readme.