[SPARK-49944][DOCS] Fix broken main.js import and fix image links for streaming documentation

[neilramaswamy]

What changes were proposed in this pull request?

We use the rel_path_to_root Jekyll variable in front of all paths that require it.

Why are the changes needed?

Currently, our import to main.js and AnchorJS are broken in the Spark 4.0.0-2 preview. Also, images aren't appearing for the Structured Streaming doc pages. See the ASF issue for more detail.

You can see how the pages are broken here; here's a screenshot, for example:

Does this PR introduce any user-facing change?

The preview documentation will now have correctly rendered code blocks, and images will appear.

How was this patch tested?

Local testing. Please build the docs site if you would like to verify.

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

No.