Add CI check to verify readme.mdx files

Follow-up on: https://github.com/elastic/integrations/issues/3260

If we decide to generate `readme.mdx` files, then we have to enable a CI mechanism to check if they are correct. We should ensure that it's enabled for all repositories with packages.

It would be nice to include it in the package- spec.

Start by reading the linked follow-up issue 3260 and inspect how the package-spec is handled. Then locate existing CI checks for package repositories and determine where readme.mdx validation should be enabled. Done means all repositories with packages verify their readme.mdx files, with the check represented in the package-spec if applicable.

Written by the indexing model from the issue text.