docs: replace self-referential Transformers guide link

[yaodong-shen]

What does this PR address?

Fixes #4541.

The Transformers API reference currently sends readers to itself as its only guide. This replaces that self-reference with the existing in-repository Hello World tutorial for a pipeline example and the model-loading guide for Hugging Face Service usage.

Validation

• pdm lock --check
• pre-commit run trailing-whitespace --files docs/source/reference/bentoml/frameworks/transformers.rst
• pre-commit run end-of-file-fixer --files docs/source/reference/bentoml/frameworks/transformers.rst
• PYENCHANT_LIBRARY_PATH=/opt/homebrew/lib/libenchant-2.dylib pdm run sphinx-build -b html docs/source /private/tmp/bentoml-docs-html (completed successfully; the repository has pre-existing warnings outside this file)

Before submitting:

• The Pull Request follows Conventional Commits naming.
• pre-commit run -a was not run because this documentation-only change has no affected Python or proto files; the applicable file hooks above passed.
• I read and followed the contribution and development guidelines.
• Documentation was updated.
• No runtime test applies to this documentation-only correction; the documentation build was run instead.