Replace images in docs/explanation/mode-of-operation.md

[asanvaq]

Images are being used in docs/explanation/mode-of-operation.md. They should be replaced with text so the content can be searchable.

[asanvaq]

The introductory paragraph should also be removed, it is not about how Chisel works.

[MChandrahas]

Hi, I have resolved the issue and made a commit. Please let me know if any changes are needed.