eclipse-mosquitto: document actual image tags

[sankalpsthakur]

Summary

Hub’s auto-generated Image Variants section implied a generic plain / -alpine split (<version> was stripped as HTML), and did not document the openssl tags. Official Mosquitto images are Alpine-based; -openssl marks the 2.0 / 1.6 OpenSSL line.

• Empty variant-alpine.md suppresses the misleading shared Alpine boilerplate
• content.md adds an Image tags section matching published supported tags

Related: eclipse-mosquitto/mosquitto#3690 / eclipse-mosquitto/mosquitto#3694

Validation

• Checked against library/eclipse-mosquitto supported tags
• Follows empty-variant-*.md pattern used elsewhere in this repo

AI/LLM disclosure

• AI coding tools (including Grok and/or Codex agent-assisted editing) were used to help draft or modify code and this PR description.
• I reviewed the complete change, understand the reasoning, and ran the reported local tests before submitting.
• This submission is original work of authorship under the project contributor terms; AI output was not pasted unreviewed.

[yosifkit]

Looks fine to me. cc eclipse-mosquitto image maintainer @ralight, does this seem ok to you?

[ralight]

That seems fine, thanks