short-term fix for para inherent weight overestimation (#5082)

closes #849

## Context

For the background on this and the long-term fix, see
https://github.com/paritytech/polkadot-sdk/issues/849#issuecomment-2247895862.

## Changes

* The weigh files are renamed from `runtime_(parachains|common).*` to
`polkadot_runtime_(parachains|common).*`. The reason for it is the
renaming introduced in #4633. The new weight command and files are
generated now include `polkadot_` prefix.
* The WeightInfo for `paras_inherent` now includes `enter_empty` which
calculates the cost of processing an empty parachains inherent. This
cost is subtracted dynamically when calculating other weights (so the
other weights remain the same)

## Benefits

See
https://github.com/paritytech/polkadot-sdk/issues/849#issuecomment-2247895862,
but TL;DR is that we are not blocked on weights for scaling the number
of validators and cores further.

Resolved questions:
- [x] why new benchmarks for westend are doing fewer db IOPS?
Is it due polkadot-sdk update (db IOPS diff)?
or the bench setup is no longer valid?

https://github.com/polkadot-fellows/runtimes/blob/7723274a2c5cbb10213379271094d5180716ca7d/relay/polkadot/src/weights/runtime_parachains_paras_inherent.rs#L131-L196
Answer: see background section of #5270 

TODOs:
- [x] Rerun benchmarks for Rococo and Westend
- [x] PRDoc

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Co-authored-by: command-bot <>