Improve visibility of "Scaling and load testing" information in DOMjudge documentation

[taoky]

Description of the problem

DOMjudge repo wiki Scaling and load testing contains important information for large contest configuration (including setting max_children value). However, this is not highlighted in DOMjudge manual. Though it gives a link to wiki in doc/manual/overview.rst, it could be neglected very easily -- As far as I know, some server admins for ICPC Asia Regional Contest have reported that they are unaware of this resource.

Your environment

• main branch of DOMjudge/domjudge

Steps to reproduce

1. View domserver installation and checklist pages:
   • https://www.domjudge.org/docs/manual/main/install-domserver.html
   • https://www.domjudge.org/docs/manual/main/quick-install.html

Expected behaviour

max_children setting (or "Scaling and load testing" wiki page) should be mentioned in relevant sections of the manual.

Actual behaviour

There is no clear mention or guidance on these critical settings for configuring large-scale contests within the main DOMjudge manual.

Any other information that you want to share?

N/A