Allow supplemental (auxiliary) content files to be fetched and imported into the content catalog

Currently, the content aggregator focuses on gathering the primary documentation content and associating it with component versions. However, the documentation doesn't necessarily own all the content it uses. A prime example is example code and example projects. This family of content is often included in the documentation, yet hosted elsewhere. It may either be in a different directory of the same branch in the repository, or it may be in a different repository. Antora already supports gathering content from multiple repositories and branches. What's different about examples is that they aren't likely to be under an Antora project structure.

Provide a means for gathering supplemental examples and enlisting those files with the content catalog. This may be as simple as allowing a content source to be marked as providing supplemental (example) files. Antora may still look for an antora.yml file in this case, but won't impose the project structure requirements. Instead, all files just get dumped into the example family as they sit.

Antora's multi-version design mandates that it know about the content it uses (meaning files must be resolved from the virtual catalog). However, this does not mean that all files, in particular example files, have to be organized using the Antora project structure. This issue should focus on example files as these are most often the files that are contributed from a different structure.

NOTE: It's possible to implement this behavior using a custom site generator. The focus of this issue is to provide this capability out of the box.