docs: Added structured EV charging use case with sequence diagrams & API mappings for Issue #2

[River-unknown]

• Added a structured use case document for EV charging, covering discovery, order placement, fulfillment, and post-charging interactions.
• Included sequence diagrams to illustrate key workflows.
• Mapped Beckn Protocol APIs to specific interactions.
• Placed the document in the docs folder as "EV-Charging-use-case.md".
• Incorporated feedback from the previous PR and addressed review comments while drafting this document.
• This PR is for the issue referenced at Create Use Case Document for EV Charging Transaction #2