FMEPRD-290 Update SEI and FF Classic FAQs

[alai97]

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Description

• Please describe your changes: Incorporates FAQ content from SEI FAQ #10161 and FAQ for FF classic #11081 into SEI and FF Classic doc sets. Adds banners for Troubleshooting SEI doc pages to specify the behavior described in these FAQs relates the SEI 1.0 experience only, not 2.0.
• Jira/GitHub Issue numbers (if any): https://harness.atlassian.net/browse/FMEPRD-290
• Preview links/images (Internal contributors only): /docs/software-engineering-insights/troubleshoot-sei/sei-widgets-faqs and /docs/faqs/harness-feature-flag-faqs

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[bot-gitexp-user]

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[santoshdodamani]

LGTM