Misc. broken links

[themaxdavitt]

The GitHub link on the sidebar points to https://github.com/huggingface/inference-endpoints, which 404s (for me at least, not sure if it's just private).

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Also, on the "Create custom Inference Handler" page, the link to "optimum" near the top of the page points to https://raw.githubusercontent.com/huggingface/optimum instead of https://github.com/huggingface/optimum.