Migration Guide for Event Hubs

[AlexGhiondea]

Please wait for this issue to be updated with the link to the Migration Guide template.

For Event Hubs, we already have a migration guide. Once the template is ready, this issue is to track the work to get the current guide to follow the template

[ramya-rao-a]

Please use the template in this PR that adds the migration guide to our policies

[yunhaoling]

• Migration benefits
  • Cross Service SDK improvements
• Important Changes
  • Client Hierarchy
  • Client Constructors
  • Sending
  • Receiving
  • Receiving with checkpoints
• Addition samples