Operate a stakepool : Hardening server guide

[Kirael12]

Checklist

• I have read the How to Contribute.
• I have run yarn build after adding my changes without getting any errors.

Updating documentation or Bugfix

I created a "Hardening Server" section which describes in 10 steps how to secure an Ubuntu server (22.04 LTS or 20.04 LTS) before installing Cardano Node :

1- Create a non-root user for your Cardano node
2- Disable root
3- Update System
4- Activate Unattended-upgrades for automatic security updates
5- Generate SSH keys
6- Hardening SSH configuration
7- Firewall configuration
8- Fail 2 ban installation and configuration
9- /etc/sysctl.conf hardening
10- Shared Memory hardening

[rdlrt]

Would be welcome addition, but just need to align as per styling guide, and maybe move the page to deployment scenarios - alongside "improve grafana security" (or in basics)

Accordingly added a few comments

[rphair]

re: 2f35a45 & #1089 (comment)

@Kirael12 we cannot include this material in the Basics section because it is not obligatory. For instance, the use of fail2ban is a popular but arbitrary choice considering all the alternatives including banning IP addresses manually (which incidentally has worked for our own pool for nearly 3 years).

Therefore @rdlrt according to our original conception of Deployment Scenarios #737 (comment) I have had to move it back there.

@Kirael12 the use of the term "deployment" has nothing to do with platforms nor containers: rather, these are methods that operators might choose to deploy in their operating environment which are not strictly required... which applies to most of the items on this new page.