AG-58384: Add backup-and-restore article and update installation guide for AdGuard for Windows v8

[SeverAnna]

Changes

New article: backup-and-restore.md

Added a comprehensive article covering backup and restore of AdGuard for Windows settings:

• How to export and import settings manually — describes the Export/Import settings buttons in App settings (.adg files)
• How to restore settings after reinstalling v8 — step-by-step guide for restoring from automatic backup ZIP files (with file descriptions: �dguard.db, �gflm_dns.db, �gflm_standard.db)
• How to switch back to v7 after updating to v8.0 — rollback instructions with link to the latest stable v7 release
• Screenshots for the settings reset notification and the uninstall dialog

Updated article: installation.md

• Fixed typos and grammar (broken sentence about first launch, capitalization inconsistencies)
• Clarified the Caution in the Advanced uninstall method
• Added restart step in the Regular uninstall method
• Updated screenshot references

Updated sidebar positions

• �pp-management/category.json: position 4 → 5
• protection/category.json: position 2 → 3
• settings/category.json: position 5 → 6
• solving-problems/category.json: position 6 → 7
• statistics/category.json: position 3 → 4

Closes AG-58384

[github-actions]

Preview was deployed to: https://pull-request-914.kb-adg.pages.dev/

[github-actions]

Preview was deployed to: https://pull-request-914.kb-adg.pages.dev/

[github-actions]

Preview was deployed to: https://pull-request-914.kb-adg.pages.dev/

[github-actions]

Preview was deployed to: https://pull-request-914.kb-adg.pages.dev/