doc: add --break-system-packages

[NicolasFR]

Description

Add --break-system-packages argument for pip in the install doc

Closes #984

Checklist:

• [X ] I have read the Contribution Guidelines
• [ X] I have performed a self-review of my own code
• [ X] I have commented my code, particularly in hard-to-understand areas
• [ X] My changes generate no new warnings

Contributor License Agreement

By submitting this pull request, I acknowledge and agree that my contributions will be included in Stirling-PDF and that they can be relicensed in the future under the MPL 2.0 (Mozilla Public License Version 2.0) license.

(This does not change the general open-source nature of Stirling-PDF, simply moving from one license to another license)

[sonarqubecloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
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[sbplat]

The PR only touches a *.md file which is ignored by the build action.