docs: add Docker quickstart

[onyx679]

Summary

• add a Docker quickstart page to the getting started guide
• document GHCR image usage, persistent config mounts, EODAG_CFG_FILE, runtime credentials, and download output mounts
• add a short README entry linking to the Docker quickstart

Closes #2185

Validation

• git diff --check
• local content assertions for the new docs page and getting started toctree

Note: I could not run the full Sphinx docs build locally because Sphinx is not installed in this Windows environment.