[KED-1564] Make dataPath relative again

[richardwestenra]

Description

Raised by @gbraccialli-qb:

"Last year, I requested Viz to work on relative paths, it's helpful to use it on jupyter hub proxy and publish it to users that don't have kedro installed locally. It was added in [the old internal version of Viz], but it doesn't work on latest version of kedro-viz. Is it possible to add it back?"

Development notes

I've added a new test to ensure that I don't break this again 🙈

QA notes

n/a

Checklist

• Read the contributing guidelines
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added new entries to the RELEASE.md file
• Added tests to cover my changes

Legal notice

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[WaylonWalker]

kedro-static-viz could be another simple option to visualize pipelines from places that do not have the pipeline installed. If you can hook it into ci and host it on GitHub pages or s3 its quite handy to have viz available anywhere. https://github.com/WaylonWalker/kedro-static-viz