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>
This PR backports version bumps from release branch
`release-polkadot-v1.6.0` back to `master` and also moved `prdoc` files
related to the release to the appropriate folder
Oliver Tale-Yazdi