Print correct webhook url when running in Spaces

[Wauplin]

When using WebhooksServer, the webhook URLs are printed in the logs so that users can configure them in their settings. However, when running in a Space the URL looks like this:

Webhooks are correctly setup and ready to use:
  - POST http://localhost:7860/webhooks/update_leaderboard
  - POST http://localhost:7860/webhooks/update_queue
Go to https://huggingface.co/settings/webhooks to setup your webhooks.

This PR fixes this by looking at the SPACE_HOST variable (if running in a Space). Logs should now show something like

Webhooks are correctly setup and ready to use:
  - POST http://open-llm-leaderboard-open-llm-leaderboard.hf.space/webhooks/update_leaderboard
  - POST http://open-llm-leaderboard-open-llm-leaderboard.hf.space/webhooks/update_queue
Go to https://huggingface.co/settings/webhooks to setup your webhooks.

cc @clefourrier who's using it for the Open LLM Leaderboard

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[Wauplin]

Thanks for the review :)