Update datagov-harvester README to remove CKAN references

[SueValente]

What

Review and update the datagov-harvester README to remove CKAN-era references and reflect current system architecture.

Why

The README contains a "Local Flask to CKAN workflow" section that references pushing datasets to a local CKAN instance, which is no longer the target system. The datagov-harvest-runner Features section also describes loading into CKAN. These references are misleading for developers doing O+M work.

Done When

• "Local Flask to CKAN workflow" section removed or updated to reflect current load target
• datagov-harvest-runner Features section updated to remove CKAN load references
• Test command updated to remove --ignore=./tests/load/ckan if CKAN load tests no longer exist
• About section added at the top describing the two applications (datagov-harvest-admin and datagov-harvest-runner) and their role in the platform
• Link to harvest.data.gov wiki page added to related resources

Notes

The rest of the README (setup, local development, deployment, Flask commands) is in good shape and does not need significant changes. This is a targeted cleanup, not a full rewrite.