Remove custom robots.txt in favor of RTD default

[DimedS]

Description

This PR removes the custom robots.txt. The file automatically generated by Read the Docs (RTD) will be used during the build.

Development notes

Developer Certificate of Origin

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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

[astrojuanlu]

It will be difficult to test this, but I'd just say let's go for it.

[noklam]

Is this ready?

[DimedS]

Is this ready?

This PR is ready, but the entire SEO optimisation isn't complete yet. If you're asking whether we should merge it before the release, I believe it's better to finish all the optimisations together. What do you think, @astrojuanlu?

[astrojuanlu]

If I understand correctly, removing robots.txt will only increase the amount of URLs we're indexing, right? So maybe we can go ahead and try the sitemap separately? #3741 (comment)