docs: standardize CLI examples and correct linkerd-cni namespace references

[bezarsnba]

This PR improves documentation consistency and correctness across the Linkerd website docs.

It includes two related documentation-only changes:

1. Standardization of CLI command examples by removing shell prompt symbols.
2. Correction of a namespace typo where linkerd-cn was used instead of the correct linkerd-cni.

These changes aim to reduce user confusion and improve copy/paste usability without altering any Linkerd behavior.

What changed

1. CLI command standardization

• Removed shell prompt prefixes (e.g. $, ~ $, / $) from bash and sh code blocks
• Ensured all command examples are copy/paste-friendly
• Applied consistently across:
  • CLI reference docs
  • Task guides
  • Troubleshooting sections
  • Edge and stable versions

2. Namespace typo fix

• Corrected incorrect namespace references:
  • linkerd-cn → linkerd-cni
• Affects CNI-related troubleshooting and validation examples
• Aligns documentation with the actual namespace used by Linkerd CNI

Why this change

• Shell prompts in docs can cause copy/paste errors and reduce clarity
• Incorrect namespace references can lead to failed commands and user frustration
• These fixes improve documentation accuracy and overall user experience

Scope

• Documentation only
• No functional, behavioral, or API changes
• No impact on Linkerd components or CLI output

Validation

• Reviewed affected pages for consistency and correctness
• Verified corrected commands align with current Linkerd conventions