Document pruning configuration

[onbjerg]

Describe the change

As we are working towards full node, we have added some new configuration. See:

• feat(stages, storage): pruning configuration #3341
• Make pruning configurable via reth.toml #3430

We should document these in the book.

Unsure if we should document them now (as they do not do anything currently), or later once we have full node. cc @gakonst

Additional context

No response

[gakonst]

Let's do it after we have the feature in cc @shekhirin u think we can be in time for alpha.3?

[shekhirin]

We also need to document how pruning of different parts affect the RPC.