AIP-76: Add example and docs for runtime asset partitioning

[anishgirianish]

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Final piece of the runtime partitioning work for AIP-76. #65447 added the producer API (PartitionAtRuntime + add_partitions) and #66782 wired up the server side, so this just adds the example DAG and docs that were promised in those PRs.

Shows a producer setting the partition key at runtime, a fan-out producer emitting several keys, and a downstream Dag consuming them via PartitionedAssetTimetable.

related: #44146 #65300

cc: @Lee-W

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.