Changes:
- Add CI script to check that the `crate` names that are mentioned in
prdocs are valid.

We can extend it lateron to also validate the correct SemVer bumps as
introduced in https://github.com/paritytech/polkadot-sdk/pull/3441.

Example output:
```pre
$ python3 .github/scripts/check-prdoc.py Cargo.toml prdoc/*.prdoc

🔎 Reading workspace polkadot-sdk/Cargo.toml.
📦 Checking 36 prdocs against 494 crates.
✅ All prdocs are valid.
```

Note that not all old prdocs pass the check since crates have been
renamed:
```pre
$ python3 .github/scripts/check-prdoc.py Cargo.toml prdoc/**/*.prdoc

🔎 Reading workspace polkadot-sdk/Cargo.toml.
📦 Checking 186 prdocs against 494 crates.
❌ Some prdocs are invalid.
💥 prdoc/1.4.0/pr_1926.prdoc lists invalid crate: node-cli
💥 prdoc/1.4.0/pr_2086.prdoc lists invalid crate: xcm-executor
💥 prdoc/1.4.0/pr_2107.prdoc lists invalid crate: xcm
💥 prdoc/1.6.0/pr_2684.prdoc lists invalid crate: xcm-builder

```

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