ci: Enforce PR limit to 3 for non-committers (#25973)

## Which issue does this PR close?

Follows [DataFusion discussion
#24413](#24413).

## Rationale for this change

See discussion for the rationale, most people are voting for 'set limit
to 3' from the discussion.

## What changes are included in this PR?

Enable `github.pull_requests.creation_cap` in `.asf.yaml` and set
`max_open_pull_requests` to `3`.

- ASF support:
[apache/infrastructure-asfyaml#111](apache/infrastructure-asfyaml#111).
- Airflow example followed by this change:
[apache/airflow#73758](apache/airflow#73758).

Whitelist support through `.asf.yaml` is being added in
[apache/infrastructure-asfyaml#135](apache/infrastructure-asfyaml#135),
which is still open. Before that I think it's possible to do some manual
close/reopen, if some contributor have > 3 concurrent PRs that is
mergeable.

## What is the testing strategy for this PR?

Should work since it's following the pattern in other ASF project above

I don't know if it's possible to test before merging

## Are there any user-facing changes?

NA

Author: 2010YOUY01

.asf.yaml

@@ -118,6 +118,13 @@ github:
     allow_auto_merge: true
     # auto-delete head branches after being merged
     del_branch_on_merge: true
+    # Limit open pull requests per user without write access.
+    #
+    # With coding agents, the number of open PRs now far exceeds our review capacity.
+    # Setting a limit helps reviewers focus on fewer PRs at a time.
+    creation_cap:
+      enabled: true
+      max_open_pull_requests: 3
 
 # publishes the content of the `asf-site` branch to
 # https://datafusion.apache.org/

docs/source/contributor-guide/index.md

@@ -91,6 +91,11 @@ committer who approved your PR to help remind them to merge it.
 
 ## Creating Pull Requests
 
+With coding agents, the number of open PRs now far exceeds our review capacity.
+To help reviewers focus on fewer PRs, users without write access are limited to
+**3 open, non-draft PRs** at a time. If you reach the limit, wait for existing PRs
+to be merged or close them until you have fewer than 3 before opening another.
+
 When possible, we recommend splitting your contributions into multiple smaller focused PRs rather than large PRs (500+ lines) because:
 
 1. The PR is more likely to be reviewed quickly -- our reviewers struggle to find the contiguous time needed to review large PRs.