add Apache Doris in README.md

[xiaokang]

Changes

Apache Doris is a modern OLAP database for real-time analytics. It delivers lightning-fast analytics on real-time data at scale. Doris is popular and its community is very active, with 13.4k github stars, 676 contributors and more than 5000 enterprise users.

One of its major user cases is logging and observability. It's already in the eco-system of OpenTelemetry as a storage backend. And the doris-opentelemetry-demo project is created to for Doris backend based on the latest offical opentelemetry-demo.

This PR try to add Apache Doris demo link in README.md.

Merge Requirements

For new features contributions, please make sure you have completed the following
essential items:

• CHANGELOG.md updated to document new feature additions
• Appropriate documentation updates in the docs
• Appropriate Helm chart updates in the helm-charts

Maintainers will not merge until the above have been completed. If you're unsure
which docs need to be changed ping the
@open-telemetry/demo-approvers.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: rogercoll / name: Roger Coll (86ac6a2)
• ✅ login: xiaokang / name: Kang (f173d2d, 7d008a2)