docs: Complete missing stage descriptions in stages.md

[VolodymyrBg]

Description:
This PR completes the documentation for all pipeline stages in docs/crates/stages.md that were previously marked as TODO. The changes include:

• Added detailed description for MerkleUnwindStage

  • Explains its role in handling chain reorganizations
  • Describes how it maintains state trie consistency
  • Documents its relationship with MerkleExecuteStage

• Added comprehensive documentation for AccountHashingStage and StorageHashingStage

  • Explains their roles in state trie construction
  • Details how they handle account and storage hashing
  • Describes their importance for state verification

• Documented MerkleExecuteStage

  • Explains its role in Merkle Patricia trie management
  • Details how it processes state changes
  • Describes state root maintenance

• Added documentation for indexing stages:

  • TransactionLookupStage: Explains transaction indexing and lookup
  • IndexStorageHistoryStage: Documents storage state history tracking
  • IndexAccountHistoryStage: Details account state history indexing

• Added documentation for FinishStage

  • Describes cleanup and verification tasks
  • Explains its role in maintaining node consistency
  • Documents metrics and status updates

These changes make the stages documentation complete and help developers better understand the pipeline architecture of Reth.

Related issue: #TODO (if there is one)

[mattsse]

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