Add a codegen attribute to list "related" Archetypes

Scalar is "related" to SeriesPoint and SeriesLine and so it would be nice to codegen appropriate cross-linking in the docs page.

Ideally we could do this with an attribute such as:
```
table Scalar (
  "attr.docs.unreleased",
  "attr.docs.related": "rerun.archetypes.SeriesLine, reruna.archetypes.SeriesPoint",
  "attr.rust.derive": "PartialEq"
) {
```