Add content_source (current/next) to assembler

[Mpdreamz]

This allows repositories to declare what content source to use for next and/or current.

Both default to main.

When checking out an environment

docs-assembler repo clone-all --environment prod

The environment configuration determines to use next or current.

Updated the checkout folder to include next or current.

The default output folder remains the same

[Mpdreamz]

The failure here is because it uses an outdated assembler but its picking up the updated src/docs-assembler/assembler.yml file vs the embedded one.

https://github.com/elastic/docs-builder/actions/runs/14200698014/job/39786692416?pr=894