Update pages for documentation generated by toStandaloneHtml with hints to LLM

Looks like indexation can be mislead by these pages, so we need to provide some backup info and explicit instruction not to look at JS, perhaps?
For old snippets it can be done here:
https://github.com/Kotlin/dataframe/blob/56140f136883bac54ab1617d34b6bc10782bf718/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/explainer/PluginCallbackProxy.kt#L229

https://context7.com/kotlin/dataframe?topic=groupBy

Start by reading core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/explainer/PluginCallbackProxy.kt around line 229, then trace how toStandaloneHtml-generated pages and old snippets are produced. Define the affected pages and update them with backup information and an explicit instruction not to inspect JavaScript; verify the generated output reflects those hints.

Written by the indexing model from the issue text.