Add Open Keep notes app to README Web UI section

[HotshotJacko1]

What does this PR do?

Add Open Keep to Web UI section

Checklist

• I have read the contribution guidelines
• The entry follows the correct format: - [icons] [Name](URL) - Description. \LICENSE` `Tech``
• The tool is not already on the list
• The tool is actively maintained (last commit within 12 months)
• Icons are accurate (checked platform support, sync, encryption)
• The link works and points to the official source
• For open-source tools: license (SPDX) and tech stack are included
• Added to the correct category