update: BYOC CLI and topic defaults

[harshini-rangaswamy]

Describe your changes

Addresses feedback for Inkless service creation docs (excluding Inkless SaaS CLI guidance, tracked separately in POD-2202).

• Fixed the BYOC CLI example to use kafka_diskless.enabled=true.
• Improved linear reading for BYOC CLI by listing available plans before the create command.
• Corrected topic defaults by separating Aiven Cloud and BYOC behavior.
• Added Inkless on Aiven Cloud create service CLI commands

POD-2229

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Checklist

• The first paragraph of the page is on one line.
• The other lines have a line break at 90 characters.
• I checked the output.
• I applied the style guide.
• My links start with /docs/.

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