Scope a grouping to specific ecosystems

toAllGroupingPages renders every grouping for every provider, so a grouping that can only be computed for one ecosystem still gets an index page and a nav link under the others, showing 0 entries.

Concretely: I group dependencies by the team that owns the code using them, joining usedBy to our service catalog. That works for the pnpm provider, where usedBy carries workspace package names. The go provider reports every module as used by a single consumer (the module itself), so nothing is attributable, and each team's page renders empty under the go tab alongside a nav link to it.

A GroupingConfig.ecosystems?: readonly string[] would cover it: when set, toAllGroupingPages skips the grouping for providers whose ecosystem isn't listed, and the nav omits it there too. Unset keeps today's behavior.

The nav part is the fiddlier half, since the grouping list for the nav is built from this.groupings in three methods (buildRows, generateDetailPage, toGroupingPages) that don't currently receive the ecosystem.

🧑‍🦲 I'll send a PR for this soon

Start by tracing toAllGroupingPages and GroupingConfig, then inspect the three navigation paths named in the issue: buildRows, generateDetailPage, and toGroupingPages. Check how this.groupings and the provider ecosystem flow through those paths. Done means ecosystem-scoped groupings produce no pages or nav links for other providers, while unset ecosystems retain current behavior.