Add documentation for how plugin CLI commands can be made lazy loaded

[ankatiyar]

Description

To go with the changes in #3883

Development notes

Added some documentation with an example of how plugin developers can make their CLI commands lazy loaded.

I extended the "simple plugin" example from the docs above but let me know if it doesn't make a 100% sense.

Developer Certificate of Origin

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Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[merelcht]

A small suggestion, but otherwise looks great! Thank you ⭐

[ElenaKhaustova]

Looks clear, nice addition! 🚀