feat(dgeni): render subpackages under root package

[griest024]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/graycoreio/daffodil/blob/develop/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Fixes: #3055

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[xelaint]

Would we be able to do something like this on the root package page?

[griest024]

Would we be able to do something like this on the root package page?

sure, but I think that should be a separate PR