docs: fix private networks setup wording

[weifanglab]

Summary

• Update the Private Networks description to refer to Kubo instead of ipfs.
• Fix the ipfs-swarm-key-gen setup sentence by removing an extra closing parenthesis.
• Use “pre-shared key” wording in the setup instructions.

Testing

• git diff --check
• rg -n 'This allows Kubo to only connect|Generate a pre-shared key using' docs/experimental-features.md

[weifanglab]

@lidel @gammazero @guillaumemichel Hi, Could you please review this PR at your convenience? Thank you very much.