Namespace URLs don't work without branch

Migrated issue

Reported by: masipila

Problem / Motivation

I'm one of the subsystem maintainers for the core Migrate system. We are polishing the Migrate API documentation as part of marking the three core migrate modules stable (Migrate, Migrate Drupal, Migrate Drupal UI).

We have an API Documentation topic at https://api.drupal.org/api/drupal/core%21modules%21migrate%21migrate.api.php/group/migration

What we would like to do there is to add links (@Link) from API overview documentation the to the source / process / destination plugin lists that are in the namespace of Drupal/migrate/Plugin/migrate/process and similarly to the source and destination plugin namespaces. The point here is that we would not need to manually maintain a list of links to these plugins but we could simply link to the namespace pages that are automatically generated. So far so good.

The problem here is that the URLs at api.drupal.org must contain the version argument (e.g. 8.5.x) when linking to the namespace pages. If you don't include the version argument in the URL, you'll get a 404 page not found.

This behavior is different compared to the URLs of individual class pages at api.drupal.org. For example, if a link points to https://api.drupal.org/api/drupal/core%21modules%21migrate%21src%21Plugin%21migrate%21process%21FormatDate.php/class/FormatDate (note that the URL does NOT have a version argument), the currently stable core version is defaulted (8.4.x at the time).

Proposed resolution

Default the current version also on the nsmespace pages so that it is possible to link to the currently stable version.

Related issue: Issue #2933620