docs: Add OpenLLM (#996)

* Add OpenLLM

* Update the order

* Separate the content into readme and doc

Signed-off-by: Sherlock113 <sherlockxu07@gmail.com>

* Update the pr

Signed-off-by: Sherlock113 <sherlockxu07@gmail.com>

* Update the pr

Signed-off-by: Sherlock113 <sherlockxu07@gmail.com>

* Update zh_cn translation

Signed-off-by: Sherlock113 <sherlockxu07@gmail.com>

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Signed-off-by: Sherlock113 <sherlockxu07@gmail.com>