feat: add Lance-HF docs to lance.org/integrations/huggingface/

[prrao87]

Add Lance-Hugging Face docs to the docs deploy workflow by checking out the external repo and copying its index.md into huggingface.md during CI, keeping Hugging Face docs sourced from the integration repo without changing local docs.

[github-actions]

PR Review

P0 Issue - Existing file will be overwritten:

The workflow copies lance-huggingface/docs/src/index.md to docs/src/integrations/huggingface.md, but this file already exists in the repository with different content. This will silently replace the existing documentation during the docs deployment.

Current content in repo (docs/src/integrations/huggingface.md):

• Shows how to write HuggingFace datasets to Lance format using lance.write_dataset

External repo content (presumed to be different based on PR description):

• Documentation from the lance-huggingface integration repo

Recommended actions:

1. Either delete the existing docs/src/integrations/huggingface.md from this repo (if the external docs are intended to replace it)
2. Or merge the content from both sources
3. Or use a different filename (e.g., huggingface-integration.md) and update the .pages nav accordingly

Without this fix, the existing HuggingFace documentation will be lost from the deployed site.

[prrao87]

It's okay to overwrite the existing docs/integrations/huggingface.md file because it's a stub. The live docs page in the external repo for the HF integration is where the docs will ultimately live.

[jackye1995]

and we can remove https://github.com/lance-format/lance/blob/main/docs/src/integrations/huggingface.md now

[prrao87]

Done!