Added -keep-going flag to build docs

[Celsuss]

NOTE: Kedro datasets are moving from kedro.extras.datasets to a separate kedro-datasets package in
kedro-plugins repository. Any changes to the dataset implementations
should be done by opening a pull request in that repository.

Description

MR for issue #3132.
Added the --keep-going flag to sphinx-build documentation in build-docs.sh (and by extension make build-docs.
Because all warnings are treated as errors the build will stop if any warnings happens, with this flag the build will still finish despite all the warnings turned errors.
This will hopefully make it a little less for contributors to worry about.

Development notes

All that was changes was the added flag --keep-going.
It's been tested by building the documentation using make build-docs with and without the flag.
With the added flag the build will still finish despite any warnings turned errors.

Checklist

• Read the contributing guidelines
• Signed off each commit with a Developer Certificate of Origin (DCO)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the RELEASE.md file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team

[stichbury]

This is great, thanks. 🏆

I put a commit in to cherry-pick in a fix to a breaking link that was impacting the RTD build.

[stichbury]

Thanks for your contribution @Celsuss 🌟

[ankatiyar]

Thank you @Celsuss for the contribution! 💯