[920] Address the review comments on the 0.4.0 Spark runtime docs

[rangareddy]

Closes #920.

What is this?

#913 added the 0.4.0 release notes and the Spark runtime quickstart. It was merged at
2026-08-25T15:20:10Z, about ten hours after a round of review comments was left on it, and those
comments were never applied. This PR applies them. No new content beyond what the review asked for.

Changes

website/docs/how-to-spark-runtime.md

Review comment	Change
Section order confuses new users; orders-job.jar appears without context	"Running a sync as its own job" now comes first, the in-job section after it
Cover building the job before running it	The in-job section is now Writing the job, Building the job, Running the job, so the code precedes the spark-submit that submits it. Building the job documents the provided-scope dependency needed to compile against the runtime classes
Other quickstarts use local paths, this one used S3	All examples now use file:///tmp/hudi-dataset/people, the table the Quickstart creates
Mention the Hudi table has to exist first	New Prerequisites section, linking the Quickstart
Add a section on running with and without a config file	XTableSparkSync now has a subsection per mode: "Syncing a single table" and "Syncing several tables from a config file"
Move the Delta paragraph into a note	It is now a :::note Which Delta implementation you get
"and newer" implies Spark 4.x	Now states Spark 3.4.x and 3.5.x explicitly (see Verification below)
The demo/spark-runtime link 404s	No change needed. It 404'd because the directory did not exist on main at review time; merging #913 created it and the link now resolves

website/releases/downloads.mdx

• Each ### Release x.y.z-incubating heading now links to its release notes page.
• Downloads moves to the top of the Releases sidebar. It rendered last because the release-notes
  pages use negative sidebar_position values (-4, -3, -2) while downloads.mdx used 1.

The existing "Release Notes:" bullet under each heading is left in place; it is now somewhat
redundant with the linked heading, and I am happy to drop it if reviewers prefer.

website/releases/release-0.4.0-incubating.mdx

• Spark version support moves from mid-sentence into a :::note callout.

Verification

Facts were read from the 0.4.0-incubating tag rather than main, because xtable-spark-runtime
exists only on the 0.4.x release line (branch-0.4 and the release branches); main's pom.xml
does not list the module.

The Spark version claim specifically:

• root pom.xml on the tag sets spark.version 3.4.2 and spark.version.prefix 3.4, with no
  3.5 anywhere in the file and no Spark profile;
• xtable-spark-runtime/pom.xml depends on hudi-spark${spark.version.prefix}-bundle and
  iceberg-spark-runtime-${spark.version.prefix};
• ITXTableSparkRuntimeBundle documents the 3.4 and 3.5 lines only.

Nothing supports Spark 4, so "3.5.x and newer" was removed.

Site checks, run locally on this branch:

• npm run build succeeds with no new warnings (the remaining ones are pre-existing blog
  truncation-marker and inline-author warnings).
• All 22 external links in the three changed files return 200.
• All 8 internal doc links resolve to a built page. Worth noting because
  docusaurus.config.js sets onBrokenLinks: 'ignore', so a broken internal link would not fail
  the build.
• Rendered output confirmed: heading order, both admonitions, the Scala/Java tabs, all nine code
  block titles, the linked release headings, and the Releases sidebar now ordered Downloads,
  0.4.0, 0.3.0, 0.2.0.

Out of scope

While verifying comments 6 and 7 I found two further inaccuracies in the same "Spark version
support" section, which this PR deliberately leaves alone so it stays scoped to the review
comments. Both are filed as #921:

1. The page says Delta Kernel is "selected automatically" on Spark 3.5 "with no flag needed".
   That is true only for the XTableSparkSync CLI, which auto-enables it via isSparkAtLeast35.
   The in-job API the page leads with has no version detection: XTableSyncService.sync reads
   spec.isUseDeltaKernel(), and TableSyncSpec.useDeltaKernel defaults to false.
2. The table labels the Spark 3.4 path "Delta Standalone", but xtable-spark-runtime/pom.xml
   depends on delta-core, delta-kernel-api and delta-kernel-defaults. It contains no
   delta-standalone dependency.