[Docs] Clarify Blaxel worker image configuration

[roomote-community]

​Opened on behalf of @mrubens. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

• documented the required BLAXEL_IMAGE value alongside the Blaxel API key and workspace for environment-based configuration
• clarified when Roomote provisions and saves the worker image automatically and when operators must provision it separately

Why this change was made

The comprehensive public-documentation audit found that the Blaxel guide omitted a runtime-required value. The controller and compute-provider factory both require BLAXEL_IMAGE, while setup can populate it only when it can resolve a registry-qualified worker image.

Impact

Self-hosted operators now have complete Blaxel configuration guidance and can distinguish the automatic setup path from the manual fallback. No product behavior changes.

[roomote-community]

No code issues found. See task

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