Migration Guide: Request-response cycle

[hacksparrow]

Document the request-response cycle differences in LB3 and LB4. Created out of #3935.

#1833 should be done before doing the task.

Acceptance Criteria

After 1833 is done,

• Go through the request response cycle in LB4 and LB3 ( with diagrams)
• Explain the differences

[dhmlau]

In yesterday's estimation, @raymondfeng suggested the following:

• start with "Document the LoopBack 4 request-response cycle" Document the LoopBack 4 request-response cycle #1833
• see if there's any LB3 documentation related to req/res cycle
• compare the two

[agnes512]

The built-in data parsing part of #4839 should be included in this story

[dhmlau]

Reopening this issue because PR #4914 only covers the LB4 portion.