Port autorest java customization doc to SDK repository

[XiaofeiCao]

Description

Port customization code from archived autorest.java repository.

Please add an informative description that covers that changes made by the pull request and link all relevant issues.

If an SDK is being regenerated based on a new swagger spec, a link to the pull request containing these swagger spec changes has been included above.

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

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[weidongxu-microsoft]

Two documentation accuracy issues to address.

[azure-pipelines]

Azure Pipelines:
Successfully started running 1 pipeline(s).
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[Copilot]

Pull request overview

This PR ports Java customization guidance (previously from the archived autorest.java repo) into azure-sdk-for-java contributor documentation, and wires it into the existing TypeSpec contributor workflow docs so developers can find it when generating/regenerating SDKs.

Changes:

• Added a new contributor guide documenting AST-based Java customization via azure-autorest-customization.
• Linked the new guide from the TypeSpec quickstart and the contributor docs index.
• Updated the SDK generation troubleshooting guide to reference the new customization documentation.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docs/contributor/typespec-quickstart.md	Adds a pointer to the new Java customization guide from the TypeSpec workflow.
docs/contributor/typespec-java-customization.md	New guide describing AST-based Java customizations using azure-autorest-customization.
docs/contributor/sdk-generation-pipeline-troubleshooting.md	Replaces the old autorest.java link with a link to the new in-repo guide.
docs/contributor/README.md	Adds the new guide to the contributor documentation index.

Suppressed comments (2)

docs/contributor/sdk-generation-pipeline-troubleshooting.md:138

• The customization-class YAML example currently uses a Java package string, but the TypeSpec docs in this repo show customization-class as a path to the customization .java file. Using the wrong form here can cause generator failures for people following this troubleshooting guide.

This issue also appears on line 134 of the same file.

> Reference: [TypeSpec Java Customization](https://github.com/Azure/azure-sdk-for-java/blob/main/docs/contributor/typespec-java-customization.md)

**Prerequisites for customization:** When adding customization code, two options must be set in `tspconfig.yaml`:
```yaml
options:

docs/contributor/sdk-generation-pipeline-troubleshooting.md:138

• This bullet describes customization-class as a Java package, which conflicts with the file-path form used elsewhere in the TypeSpec Java customization docs and can mislead readers configuring tspconfig.yaml.

> Reference: [TypeSpec Java Customization](https://github.com/Azure/azure-sdk-for-java/blob/main/docs/contributor/typespec-java-customization.md)

**Prerequisites for customization:** When adding customization code, two options must be set in `tspconfig.yaml`:
```yaml
options: