Adding migration instruction from benchmarking v1 to v2 (#6093)

# Adding instruction to migrate benchmarking from v1 to v2

Even if the documentation for benchmarking v1 and v2 is clear and
detailed, I feel that adding a migration guide from v1 to v2 would help
doing it quicker.

## Integration
This change only affects documentation, so it does not cause integration
issues.

## Review Notes
I followed the migration procedure I applied in PR
https://github.com/paritytech/polkadot-sdk/pull/6018 . I added
everything from there, but I may be missing some extra steps that are
needed in specific case, so in case you notice something please let me
know.

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Co-authored-by: Dónal Murray <[email protected]>