docs: use gpt-4o-mini in OpenAIGenerator and DeepEvalEvaluator examples

[camgrimsec]

Replace 'gpt-4' with 'gpt-4o-mini' in the basic-usage examples for OpenAIGenerator and DeepEvalEvaluator on the docs website.

Rationale:

• gpt-4o-mini is OpenAI's current cost-efficient default — $0.15/$0.60 per M tokens vs gpt-4 at $30/$60 (~200x cheaper input, ~100x cheaper output).
• The surrounding prose (e.g. 'OpenAIGenerator supports OpenAI models starting from gpt-3.5-turbo and later (gpt-4, gpt-4-turbo, and so on)') remains accurate — these examples are not contracts about which model the user must pick, only what a copy-pasted first run will cost.
• Users running the doc snippet verbatim today incur ~200x more cost than needed for a 'hello world' generator example.

No code changes. Component constructors remain unchanged — the user explicitly passes the model name. Docs only.

Related Issues

• fixes #issue-number

Proposed Changes:

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• I have run pre-commit hooks and fixed any issue.

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