Update Admin Guide page

[hassanakbar4]

component_MailArchive: User Interface type_task | by rcross@amsl.com

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• add new "backend" subdirectory to paths where required
• update celery management section

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Issue migrated from trac:2825 at 2021-09-22 16:56:07 +0500

[hassanakbar4]

@hassanakbar4 changed status from new to accepted