docs: mark Meta Llama integration as discontinued

[Ricky-7-Yan]

Related Issues

• Addresses the remaining Haystack documentation item in Remove the meta-llama integration haystack-core-integrations#3544

Proposed Changes

• add a prominent discontinuation warning to MetaLlamaChatGenerator
• explain that Meta shut down the public preview Llama API on July 6, 2026
• link readers to supported Llama alternatives
• replace the deleted main source link with the final pre-removal source revision
• apply the correction to both next and current stable (3.1) docs, including the generator index

How did you test it?

• verified all four suggested alternative component pages exist in both documentation versions
• confirmed the pinned historical source link returns HTTP 200
• checked the changed MDX admonition delimiters and current/stable parity
• a full Docusaurus build was not run because the extracted source tree has no installed Node dependencies

Notes for the reviewer

This is documentation-only, so no release note or unit tests are included. The integration code was removed in deepset-ai/haystack-core-integrations#3552 after the backing API shutdown.

This PR was prepared with an AI assistant. The changed text, upstream removal context, and link targets were checked before submission.

Checklist

• I have read the contributor and documentation guidelines
• I have linked the related issue and upstream removal PR
• The title uses a conventional commit type
• No release note is required for a docs-only change

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