Revise link to notebook docs and remove unnecessary intro page

[stichbury]

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

I realised the link in the introductory material was to the original notebook content rather than the newer and more useful index page. In fixing that, I further realised that the introduction/introduction.html page was somewhat unnecessary and the material could simply go onto introduction/index.html. So this fixes this. We need to add a redirect when released.

Development notes

Updated link, updated page (removed page).

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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]

Preview page is here: https://kedro--3244.org.readthedocs.build/en/3244/introduction/index.html

[astrojuanlu]

YES!

[merelcht]

LGTM 👍