Archive the staged integration instructor-embedders

[julian-risch]

We staged the integration instructor-embedders in July 2025 in PR #2079 and after this extended grace period, we should archive the integration now.
That means we should:

• Add a warning to the readme file of the integration similar to other archived integrations.
• Update the status of the integration in https://github.com/deepset-ai/haystack-core-integrations/tree/main?tab=readme-ov-file#inventory
• Make a final release 0.4.1.post1 of the integration to sync the README.md file with the one shown on pypi. A maintainer can help with that.
• Remove the source code of the integration from the staging branch
• Add a warning to the integrations page https://haystack.deepset.ai/integrations/instructor-embedder
• Remove https://github.com/deepset-ai/haystack-integrations/blob/main/integrations/instructor-embedder.md

Additional context
Our policy for discontinuing an integration is here: https://docs.haystack.deepset.ai/docs/breaking-change-policy#discontinuing-an-integration

[anakin87]

We might want to remove the integration from README and integrations as we did in the past with Uptrain: deepset-ai/haystack-integrations#342