chore: add description for zksync_settlement_layer_data (#4425)

## What ❔

Add description field for `zksync_settlement_layer_data`.

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## Why ❔

To fix crates.io publishing for `core`.

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## Is this a breaking change?
- [ ] Yes
- [x] No

## Checklist

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- [x] PR title corresponds to the body of PR (we generate changelog
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- [x] Tests for the changes have been added / updated.
- [x] Documentation comments have been added / updated.
- [x] Code has been formatted via `zkstack dev fmt` and `zkstack dev
lint`.

Author: antonbaliasnikov

core/lib/settlement_layer_data/Cargo.toml

@@ -1,5 +1,6 @@
 [package]
 name = "zksync_settlement_layer_data"
+description = "ZKsync settlement layer data"
 version.workspace = true
 edition.workspace = true
 authors.workspace = true