Add star and contributor thresholds to "Known Users" inclusion criteria #26803

Context

Follow up to this @alamb's comment

Problem

The "Known Users" active list in docs/source/user-guide/introduction.md
uses one criterion, written only as an HTML comment:

<!-- "Active" means github repositories that had at least one commit in the last 6 months -->

That criterion alone has two gaps:

• It is not enforced. As of 2026-10-27 (cutoff 2026-04-27), five listed
  entries already fail it.
• It says nothing about adoption. A brand-new, single-author repo qualifies
  as soon as it has a commit. KalamDB (0 stars, 1 contributor, created
  2026-06-23) is listed alongside InfluxDB (31.8k stars, ~530 contributors).

Why it matters

The list reads as an endorsement of production-grade DataFusion users.
Stale or very small entries weaken that signal and make it harder to judge
PRs that add links. Without objective thresholds, every addition becomes a
judgement call for reviewers.

Invariant / desired behavior

An entry in the active list must link to a public GitHub repo that meets
all of the following:

1. At least 1 commit in the last 6 months (existing rule).
2. At least N stars (proposed: 100).
3. At least M contributors (proposed: 5).

The criteria are stated in visible doc text, not only in an HTML comment.
Entries without a public repo follow an explicitly documented rule (see open
questions).

Current data (GitHub, 2026-10-27)

Stars come from the GitHub API. Contributors are the count shown in each
repo's GitHub sidebar ("Contributors N"), read on 2026-10-27. The REST
contributors endpoint is not used: it consistently undercounts (for
example, GlareDB 5 vs 19 and InfluxDB 314 vs 530), and with anon=1 it
double-counts people who commit from several emails.

No public GitHub repo (stars/contributors not measurable): datapress,
Funnel, hiop, Hotdata, IceGate, Massive.com, R2 Query Engine, Streamling,
Supermetal, Synnada, Telemetry.

Observations

• Already fail the 6-month rule: ROAPI, HoraeDB (archived), GlareDB,
  Seafowl (backs the EDB Postgres Lakehouse entry), VegaFusion.
• About to fail: dbt Fusion (last commit 2026-06-04, about 5 weeks from
  the cutoff).
• Clear outlier: KalamDB (0 stars, 1 contributor, repo created
  2026-06-23).
• Stars among active repos are mostly in the hundreds to thousands; the
  lowest are Sleeper (107), Infino (138), datafusion-dft (205).
• Low contributor counts (<10): ArkFlow (8), datafusion-dft (8), qv (7), all
  active.

Threshold impact

Stale links found

• Lance: lancedb/lance → lance-format/lance
• HoraeDB: apache/incubator-horaedb → apache/horaedb (archived)
• Restate: links to the org restatedev, not the repo restatedev/restate

Proposed direction

1. Replace the HTML comment with a short visible paragraph that lists the
   three criteria and the exemption rule for entries without a public repo.
2. Apply the criteria to the current list: move failing entries to "less
   active projects", and remove or move entries below the thresholds.
3. Fix the stale links above.
4. Optional: commit a small script (for example under dev/) that prints
   stars, contributors and last commit for every GitHub link in the section,
   so the list can be re-audited periodically.

Scope

In

• Inclusion criteria text in docs/source/user-guide/introduction.md.
• Re-categorizing current "Known Users" entries against the criteria.
• Fixing the stale repo links listed above.

Out

• The "Integrations and Extensions" / "Language Bindings" lists, unless
  maintainers want the same criteria there (follow-up).
• Automated CI enforcement of the thresholds.
• Judging projects on anything beyond the stated criteria.

Open questions

• Final values for N (stars) and M (contributors). Proposed: 100 / 5.
• Entries without a public repo (11 today): exempt them (e.g. as
  "commercial products using DataFusion"), list them separately, or require
  some other evidence of use (a blog post or docs page).
• Should entries that fail the thresholds go to "less active projects" or be
  removed entirely?

Acceptance criteria

• introduction.md states the activity, star and contributor criteria
  in visible text.
• The rule for entries without a public GitHub repo is documented.
• Every GitHub-linked entry in the active list meets all criteria as of
  the PR date.
• ROAPI, HoraeDB, GlareDB, Seafowl/EDB Postgres Lakehouse and
  VegaFusion are no longer in the active list (unless they regain
  activity before merge).
• Lance, HoraeDB and Restate links point to the current repos.