fix(doc): add enable systemctl for centreon

[lpinsivy]

Description

Add in documentation how to enable systemctl for centreon systemd process

Fixes #7273 #7246

Type of change

Please delete options that are not relevant.

• Patch fixing an issue (non-breaking change)
• New functionality (non-breaking change)
• Breaking change (patch or feature) that might cause side effects breaking part of the Software
• Updating documentation (missing information, typo...)

Target serie

Please delete series that are not relevant.

• 2.8.x
• 18.10.x
• 19.04.x (master)

How this pull request can be tested ?

Please describe the procedure to verify that the goal of the PR is matched. Provide clear instructions so that it can be correctly tested.

Any relevant details of the configuration to perform the test should be added.

Checklist

Community contributors & Centreon team

• I followed the coding style guidelines provided by Centreon
• I have commented my code, especially new classes, functions or any legacy code modified. (docblock)
• I have commented my code, especially hard-to-understand areas of the PR.
• I have made corresponding changes to the documentation.
• I have rebased my development branch on the base branch (master, maintenance).

Centreon team only

• I have made sure that the unit tests related to the story are successful.
• I have made sure that unit tests covers 80% of the code written for the story.
• I have made sure that acceptance tests related to the story are successful (local and CI)