docs: add feature flags in E2E tests section

[Unik0rnMaggie]

• Add a new Feature flags in E2E tests section to the shared E2E testing guidelines.
• Document the two categories of feature flags (remote/runtime and build-time/compile-time) with guidelines and examples for each.
• Include general principles for testing both flag states, keeping flag logic explicit in helpers, and cleaning up flags after rollout.

Fixes: MMQA-1481

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Note

Low Risk
Low risk documentation-only changes; no runtime or test code behavior is modified.

Overview
Adds a new Feature flags in E2E tests section to docs/testing/e2e-testing.md, clarifying how to test remote/runtime vs build-time/compile-time flags, including recommended override patterns and examples.

Updates docs/remote-feature-flags.md to explain Extension’s production remote-flag fetch and E2E behavior via the feature flag registry, and replaces the embedded E2E override snippet with a link to the new E2E feature-flag guidelines.

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[DDDDDanica]

Thanks for adding the documentation, LGTM !