Update custom data skill template

[Ruuudy1]

Moves and reworks the custom-data skill work from QuantConnect/Agent-Skills-and-Workflows#18 into the Documentation skill-template convention.

Changes:

• Updates skill-templates/importing-data/custom-data/SKILL.md in place.
• Keeps the template self-contained for the Documentation bundler.
• Uses the Lean CLI directly for Cloud Object Store uploads with lean cloud object-store set <key> <path>.
• Keeps dual Python and C# guidance through py / cs markers.

Validation:

• python skill-templates\\bundle-skills.py --validate-only
• python skill-templates\\bundle-skills.py --dry-run
• Checked the changed file for upload-object-store references, em dashes, non-ASCII punctuation, and non-ASCII characters.