[Documentation] Update README after Bolt backend was merged into Apache Gluten

### Issue Type

Typo / Grammar mistake

### Location (URL or File Path)

README

### Description of the Issue

### Issue Type

Outdated information

### Location (URL or File Path)

`README.md`

Affected sections:

- `Embrace the Analytical Ecosystem`
- `Building Bolt for Gluten`

### Description of the Issue

The Bolt backend has been merged into the Apache Gluten main branch through [apache/gluten#12454](https://github.com/apache/gluten/pull/12454), but the README still describes the pre-merge development workflow.

Specifically:

- The Apache Gluten link points to an old discussion under `apache/incubator-gluten`.
- The build instructions clone the personal fork `WangGuangxin/gluten`.
- The instructions depend on the temporary `add_bolt_backend` branch.
- The Gluten build command still uses the outdated generic `make jar` target.

These instructions no longer reflect the current integration status or the build process in the official Apache Gluten repository.

### Suggested Fix (Optional)

pdate the README to:

- Link to the official [Apache Gluten Bolt Backend documentation](https://github.com/apache/gluten#bolt-backend).
- State that Bolt is now available as a backend in Apache Gluten.
- Reference the merged integration PR, [apache/gluten#12454](https://github.com/apache/gluten/pull/12454).
- Replace the personal fork and temporary branch with the official `apache/gluten` repository.
- Update the Gluten packaging example to use a Spark-version-specific target, such as `make jar_spark35`.
- Retain the instructions for exporting local Bolt changes to the Conan cache so developers can validate them with Gluten.

### Additional Context

_No response_