doc: fixed references to Haystack and LangChain documentation in README

[kumarlokesh]

What's being changed:

In the readme file, Weaviate usage links have been updated for:

1. Haystack

• Previous link (https://docs.haystack.deepset.ai/reference/document-store-api#weaviatedocumentstore)
  redirected to a page with 404 page not found message.
• Updated link to https://docs.haystack.deepset.ai/reference/integrations-weaviate#weaviatedocumentstore.

2. LangChain

• Previous link (https://python.langchain.com/docs/ecosystem/integrations/weaviate)
  redirected to a page with Page Not Found message.
• Updated link to https://python.langchain.com/docs/integrations/providers/weaviate.

Review checklist

• Documentation has been updated, if necessary. Link to changed documentation:
• Chaos pipeline run or not necessary. Link to pipeline:
• All new code is covered by tests where it is reasonable.
• Performance tests have been run or not necessary.

[sonarcloud]

Quality Gate failed

Failed conditions
5.0% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud