[policy, no patch] Allow experimental core features in addition to experimental modules

Problem/Motivation

Experimental modules have been used to successfully introduce new features in Drupal core. In the case of SDC, the transition from beta module to stable feature was quite tedious #3352256: Move code from the experimental SDC module to core.

The core mailer project did take a different approach because of that experience. An alpha level experimental module was committed to core, but it only contains the services yml file. The implementation itself is placed directly into lib/Drupal/Core/Mailer. As a result, those classes do not need to be renamed when the experimental module transitions to a stable feature.

This approach could be streamlined by not requiring a module at all. The services yaml file of an experimental feature could be placed somewhere in the source tree. In order to enable that feature, users need to add the path to that file via container_yaml setting. This approach is explored in #3542264: Rename mailer module to mailer_symfony and avoid namespace conflict with contrib mailer module

Steps to reproduce

Proposed resolution

Add policy documentation for this approach.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet