Add documentation how to use the OpenAI client to access DeepSeek

[apappascs]

DeepSeek has recently garnered significant attention in the global AI community. This pull request introduces comprehensive documentation and integration tests to facilitate its use with Spring AI.

[apappascs]

following the examples from @tzolov on Nvidia and Groq

[apappascs]

Pricing Comparison: DeepSeek vs OpenAI

DeepSeek-Chat vs GPT-4o

Feature	DeepSeek-Chat	GPT-4o	Percentage Difference (Cheaper)
1M Input Tokens (Cache Miss)	$0.14	$2.50	94.4%
1M Cached Input Tokens	$0.014	$1.25	98.88%
1M Output Tokens	$0.28	$10.00	97.2%

---

DeepSeek-Reasoner vs o1

Feature	DeepSeek-Reasoner	o1	Percentage Difference (Cheaper)
1M Input Tokens (Cache Miss)	$0.14	$15.00	99.07%
1M Cached Input Tokens	$0.55	$7.50	92.67%
1M Output Tokens	$2.19	$60.00	96.35%

[markpollack]

thanks, pretty neato!

merged in bada403