Config activity log as a post-installation step

[adriendupuis]

Question	Answer
JIRA Ticket	N/A
Versions	4.6, master
Edition	All

At the end of installation process, suggest to configure activity log to limit its length. I add "Configure recent activity log" section before the two other cron related sections because it can be critical to fill up DB with activity log while the other two are just feature enablements.

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• docs/getting_started/install_ibexa_dxp.md