Add Securing nodes page

[blukat29]

Proposed changes

• Add the "Securing nodes" page under the Nodes > Core Cell guides.
• It's about general security advice for node operators.

Note to maintainers:

• I tried to build the docs but failed with a "Hook useDoc is called outside the <DocProvider>" error. Therefore I am only submitting a markdown file.
• I want to locate this page in the Core Cell section. Teach me how or configure yourself, please.

  Nodes]
  ore Cell
  System Requirements
  Network Configuration
  Install Core Cell
  Monitor Core Cell
  Configure High Availability
  Securing Nodes <-- here
  

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