Document YAML approach for custom connection form fields

[amoghrajesh]

After: #60410 and ongoing community effort via: #62345, connection UI metadata (custom fields and field
behaviour) can be defined declaratively in provider.yaml instead of Python hook methods for providers.

Custom providers written with the Python approach continue to work unchanged but we should update the docs
for community reference.

In this PR, I am updating the docs to show the YAML approach as preferred, with working examples for conn-fields and ui-field-behaviour, while preserving the Python examples under a clearly labeled "legacy" section.

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[eladkal]

Nice!