build: add examples module on a multi-module Maven build (#32)

## Which issue does this PR close?

- Closes #.

## Rationale for this change

The repository lacks runnable end-to-end examples. Code snippets in the
docs are easy to drift out of sync with the API: there is no build step
that fails when a public method is renamed or removed.

Adding an `examples/` Maven module that depends on the library lets the
reactor compile every example on each build, so they cannot fall behind
the API. Doing this requires the repo to be a multi-module Maven
project; while we're there, the parent POM gets shared
`dependencyManagement` and plugin versions so child modules stay terse.

## What changes are included in this PR?

**Multi-module restructure:**

- Root `pom.xml` becomes the parent (`datafusion-java-parent`,
`packaging=pom`) with `dependencyManagement` for arrow, protobuf, junit,
and the library itself.
- `core/` is a new directory holding the existing library
(`datafusion-java`). `src/` moves to `core/src/`.
- `examples/` is a new module (`datafusion-java-examples`) depending on
the library via `${project.version}`. It wires `exec-maven-plugin` so
each example launches with the right `java.library.path` and
`--add-opens` flags.
- `native/`, `proto/`, `Makefile`, and `mvnw` stay at the repo root
unchanged.
- Surefire's `java.library.path` now uses
`${maven.multiModuleProjectDirectory}` so it resolves under the reactor
regardless of which module Maven is invoked from.
- `apache-rat-plugin` runs only at the root
(`<inherited>false</inherited>`); the rat exclude list is unchanged.

**Three runnable examples** under
`examples/src/main/java/org/apache/datafusion/examples/`:

- `SqlQueryExample` — `registerCsv` + a SQL `GROUP BY` aggregation.
- `DataFrameExample` — `readCsv` → `filter` / `select` /
`withColumnRenamed` / `distinct` → `writeParquet(singleFileOutput)` →
`readParquet` round-trip.
- `ProtoPlanExample` — build a `LogicalPlanNode` directly via the
generated protobuf classes and execute it through
`SessionContext.fromProto`.

Each example creates its own throwaway data in a temp dir and cleans up,
so no external fixtures (TPC-H, etc.) are required.

**Docs:** `docs/source/contributor-guide/development.md` is updated with
the new repo layout and a "Running an example" section that documents
the `./mvnw install -DskipTests` + `exec:exec` flow.

## Are these changes tested?

- The full JVM test suite (`./mvnw test`) still passes against the
relocated `core/src/test/` sources — 61 tests run, 0 failures (12
skipped, same skip pattern as `main` when TPC-H data is absent).
- Each example was executed end-to-end via `./mvnw -pl
:datafusion-java-examples exec:exec` and produces expected output:
  - `SqlQueryExample` prints `HIGH 3 215 / MEDIUM 1 60 / LOW 1 25`.
- `DataFrameExample` prints a 3-row deduped table and `Round-tripped row
count: 3`.
  - `ProtoPlanExample` prints `42  7`.
- `spotless:check` and the reactor build are clean across all three
modules.

## Are there any user-facing changes?

- The published library artifact
(`org.apache.datafusion:datafusion-java`) is unchanged — same `groupId`,
`artifactId`, `version`, and package contents.
- The repo layout changes: source paths move from `src/main/java/...` to
`core/src/main/java/...`. IDE projects pointing at the old location need
to re-import.
- New `datafusion-java-examples` artifact exists but is marked
`maven.install.skip=true` / `maven.deploy.skip=true` and is not intended
for distribution.