docs: add Data and Privacy section explaining telemetry usage

Description

we publish a gitlab plugin on Open VSX, and we received the attached email below about updates in their terms

• Original legal review https://gitlab.com/gitlab-com/legal-and-compliance/-/issues/330

• Current legal review gitlab-com/legal-and-compliance#3028

• The published changes 

• Slack thread: https://gitlab.slack.com/archives/C78E74A6L/p1760714899133129

• Quote from the email:

  By continuing to use the registry after 17 October 2025, you agree to the updated terms effective 30 days after this notice

According to https://gitlab.com/gitlab-com/legal-and-compliance/-/issues/3028#note_2836065817 we should add the following text to our Open VSX listing which is basically just our extension readme

GitLab Workflow extension uses the telemetry settings in Visual Studio Code to send usage and error information to GitLab. See this page for instructions on how to enable or customize telemetry in Visual Studio Code. The processing of any personal data is in accordance with our Privacy Statement.

Related Issues

Resolves #2147 (closed)

How has this been tested?

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.
• Consider an end-to-end test for significant new features that aren't covered by integration tests.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes
• docs: add Data and Privacy section explaining telemetry usage

Closes #2147 (closed)