docs: Updated integration doc for aleph alpha (#16844)
Description: Updated doc for llm/aleph_alpha with new functions: invoke.
Changed structure of the document to match the required one.
Issue: https://github.com/langchain-ai/langchain/issues/15664
Dependencies: None
Twitter handle: None
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Co-authored-by: Radhakrishnan Iyer <radhakrishnan.iyer@ibm.com>
