docs(user-guide): document feature flags migration to backend config

[kostiantyn-pshenychnyi1]

Summary

• Documents migration of features:userManagement, features:budgetManagement, features:enterpriseEdition, and features:showAllProjects from frontend env vars to backend customer-config.yaml (EPMCDME-12746)
• Updates Platform-managed Mode Configuration guide to reference the new customer-config.yaml flags instead of removed viteEnableUserManagement/viteEnableBudgetManagement Helm values
• Adds all four new flags to the Customer Feature Configuration reference with accurate descriptions

Changes

• docs/admin/configuration/access-control/platform-managed-mode-configuration.md — replaced UI Helm values section with features:userManagement and features:budgetManagement in customer-config.yaml; updated warning block
• docs/admin/configuration/codemie/customer-feature-configuration.md — added features:userManagement, features:budgetManagement (PLATFORM-MANAGED MODE section), features:enterpriseEdition (Advanced Features), and features:showAllProjects (CORE FEATURES) to overview table, YAML examples, and full config example
• faq/how-do-i-enable-project-and-user-management-in-codemie.md — replaced viteEnable* Helm values with features:userManagement/features:budgetManagement in customer-config.yaml

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Relates to MRs:

• codemie-ui !1388 — migrates feature flag checks to backend config, removes viteEnableUserManagement/viteEnableBudgetManagement
• codemie !3580 — adds four new feature flag entries to customer-config.yaml