main: new script to start a release

[ralfhandl]

New script start-release.sh and instructions in CONTRIBUTING.md on how to use it to start

• a patch release, only initializing src/oas.md with fresh history and bumped version
• a minor or major release, also bumping the version in all schema files and tests

The script has been tested to initialize

• 3.1.3 in this repo - 3.1: initialize src/oas.md for patch release 3.1.3 #5043
• 3.2.1 in this repo - 3.2: initialize src/oas.md for patch release 3.2.1 #5074, cherry-picking all changes to src/oas.md since the release of 3.2.0
• 3.3.0 in this repo - 3.3: initialize for minor release 3.3.0 #5044
• 4.0.0 in my fork - diff with v4.0-dev branched off v3.3-dev

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• no schema changes are needed for this pull request