PSMDB-1284 Doc: File copy based initial sync

[nastena1606]

new file: docs/initial-sync.md modified: docs/psmdb-pro.md modified: mkdocs-base.yml

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[radoslawszulgo]

I'd change the file name to fcbis-initial-sync.md or something. As there's also logical initial sync - the name of the file (and the URL) might be misleading.