docs(java): fix the installation section structure (#2521)

Two sentences told the reader to replace `${copilot.sdk.version}`, but no
snippet contains that placeholder - the release automation writes concrete
versions in. The copy under the snapshot Gradle example additionally said to
use "the latest release from Maven Central" while showing a -SNAPSHOT.

`Snapshot Builds` was also nested under `### Gradle` even though it covers
both build tools, and the snapshot Gradle example sat at `###`, so the page
rendered two sibling "Gradle" installation sections with conflicting versions
(and a duplicate `gradle` heading slug).

Promote the section to `### Snapshot builds` with `#### Maven` / `#### Gradle`
underneath, and drop the stale sentences. No version string changes, so
scripts/update-documentation-versions.sh keeps its one-release-and-one-snapshot
contract for both build tools.

Co-authored-by: Zhangyi Yuan <j-zhangyiyuan@microsoft.com>

Author: rinceyuan

java/README.md

@@ -30,8 +30,6 @@ runtime.
 
 ### Maven
 
-Replace `${copilot.sdk.version}` with the latest release from Maven Central.
-
 ```xml
 <dependency>
     <groupId>com.github</groupId>
@@ -46,9 +44,11 @@ Replace `${copilot.sdk.version}` with the latest release from Maven Central.
 implementation 'com.github:copilot-sdk-java:1.0.13-preview.5'
 ```
 
-#### Snapshot Builds
+### Snapshot builds
+
+Snapshot builds of the next development version are published to Maven Central Snapshots. To use them, add the snapshot repository and depend on the development version:
 
-Snapshot builds of the next development version are published to Maven Central Snapshots. To use them, add the repository and update the dependency version in your `pom.xml`:
+#### Maven
 
 ```xml
 <repositories>
@@ -66,9 +66,7 @@ Snapshot builds of the next development version are published to Maven Central S
 </dependency>
 ```
 
-### Gradle
-
-Replace `${copilot.sdk.version}` with the latest release from Maven Central.
+#### Gradle
 
 ```groovy
 implementation 'com.github:copilot-sdk-java:1.0.14-preview.5-SNAPSHOT'