[WIP][SPARK-56015][INFRA][DOCS] Cleanup docs container, remove unused R deps, and fix x86 build.

[holdenk]

What changes were proposed in this pull request?

Update the documentation build README, container, and steps. Takes over from #54838

Why are the changes needed?

• SparkR is deprecated and docs are built outside the container anyways
• Current docs build is hard coded to assume ARM
• For building the deprecated R docs people need to install an extra package
• We shouldn't be creating a bunch of files as root in the users spark directtory if we can avoid it.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Built docs docker container locally, built docs with the doc build shell script locally, built website with docs container locally.

Was this patch authored or co-authored using generative AI tooling?

• Asked chat gpt about the different CRAN install methods.
• copilot in autocomplete mode for some of it
• Various asking claude about R & Jekyll stuff I'm not super up to date on
• case/switch in shell script generated by claude