docs: add Bazel build guide and bzlmod example

[cw20050111-prog]

What problem does this PR solve?

Issue Number: resolve apache#2991

Problem Summary:

The build documentation currently focuses on config_brpc.sh and CMake. It does not provide a clear Bazel/bzlmod build path for users who want to build bRPC or use bRPC as a Bazel dependency.

What is changed and the side effects?

Changed:

• Add Bazel/bzlmod build instructions to the English and Chinese getting started docs.
• Rewrite the English and Chinese Bazel support docs with bzlmod-first dependency guidance.
• Document the required registries and root MODULE.bazel setup for using bRPC as a Bazel dependency.
• Add an echo client target to example/build_with_bazel_module so the bzlmod example has both server and client targets.
• Add required leveldb and openssl single_version_override entries for aarch64 bzlmod builds, and document that users must repeat these overrides in their own root MODULE.bazel.

Side effects:

• Performance effects: None. This PR only changes documentation and the Bazel example.

• Breaking backward compatibility: None.

Validation:

• GitHub Actions passed on PR docs: add Bazel build guide and bzlmod example #13.
• Linux x64: Bazel all-options jobs passed with both gcc and clang, including the external bzlmod example under example/build_with_bazel_module.
• macOS ARM64: Bazel build passed.
• Linux aarch64: manually verified with cd example/build_with_bazel_module && bazel-7.2.1 $JVMARG build //:echo_c++_server //:echo_c++_client.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.