[prdoc] Validate crate names (#3467)

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

```

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

.github/workflows/check-prdoc.yml

@@ -57,3 +57,11 @@ jobs:
           echo "Checking for PR#${GITHUB_PR}"
           echo "You can find more information about PRDoc at $PRDOC_DOC"
           $ENGINE run --rm -v $PWD:/repo -e RUST_LOG=info $IMAGE check -n ${GITHUB_PR}
+
+      - name: Validate prdoc for PR#${{ github.event.pull_request.number }}
+        if: ${{ !contains(steps.get-labels.outputs.labels, 'R0') }}
+        run: |
+          echo "Validating PR#${GITHUB_PR}"
+          python3 --version
+          python3 -m pip install cargo-workspace==1.2.1
+          python3 .github/scripts/check-prdoc.py Cargo.toml prdoc/pr_${GITHUB_PR}.prdoc