Update pages for documentation generated by toStandaloneHtml with hints to LLM

[koperagen]

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:

dataframe/core/src/test/kotlin/org/jetbrains/kotlinx/dataframe/explainer/PluginCallbackProxy.kt

Line 229 in 56140f1

private fun convertToHtml(dataframeLike: Any): DataFrameHtmlData {

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