[Merged by Bors] - Adds note about external components in the demos that we do not support

[soenkeliebau]

Description

We need to be careful to distinguish between components that we do and do not support, and in the demos provisioned by stackablectl we quite happily mix "external" and "internal" components.

This PR adds a note to the docs about this.

Please add a description here. This will become the commit message of the merge request later.

Review Checklist

• [ ] Code contains useful comments
• [ ] (Integration-)Test cases added (or not applicable)
• [ ] Documentation added (or not applicable)
• [ ] Changelog updated (or not applicable)
• [ ] Cargo.toml only contains references to git tags (not specific commits or branches)

Once the review is done, comment bors r+ (or bors merge) to merge. Further information

[soenkeliebau]

bors r+

[bors[bot]]

Pull request successfully merged into main.

Build succeeded:

• Run Rustfmt