[docs] Add diagram to "Lifecycle methods" section

#### Support plan

Community

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): no

#### Context

* *node version*: irrelevant
* *module version*: 20
* *environment* (e.g. node, browser, native): irrelevant
* *used with* (e.g. hapi application, another framework, standalone, ...): irrelevant
* *any other relevant information*: irrelevant

#### What problem are you trying to solve?

Detailed description of [request lifecycle](https://hapi.dev/api/?v=20.0.0#request-lifecycle) is rather complicated and it's hard to track flow.

#### Do you have a new or modified API suggestion to solve the problem?

Add flow diagram explaining request lifecycle.