[prdoc] Require SemVer bump level

[liamaharon]

A prerequisite for adding a stable branch and respecting SemVer on new stable releases is including SemVer bump levels in our PRDocs.

Next release is scheduled for April 3rd, so it would be great to get this merged before then.

Also added "None" as a valid bump option, to support test/benchmark changes and CI to ensure changed crates have an entry.

[ggwpez]

@bkchr are you ready for this 🤣 ? We still need a script that checks that the Cargo.toml files are in sync (working on it).

[kianenigma]

We will never be fully ready for this until we merge it and force ourselves to be ready, nos vamos 🤠

[liamaharon]

bot fmt

[command-bot]

@liamaharon https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5669798 was started for your command "$PIPELINE_SCRIPTS_DIR/commands/fmt/fmt.sh". Check out https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/pipelines?page=1&scope=all&username=group_605_bot to know what else is being executed currently.

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[command-bot]

@liamaharon Command "$PIPELINE_SCRIPTS_DIR/commands/fmt/fmt.sh" has finished. Result: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5669798 has finished. If any artifacts were generated, you can download them from https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/5669798/artifacts/download.