Add Config API documentation & guide

[nirrozenbaum]

Config API documentation & guide should include:

• explanation about the structure, versioning, etc (make it clear that it is not a CRD, not changeable at runtime, etc.).
• how to share same instance across extension points, including default behaviors (new instance if not mentioned otherwise), etc.
• valid config examples (could be just pointers to our usage once it's used in main).
• how to add and register out-of-tree plugins (could be pointers to llm-d?).
• recommendations (e.g., use my-plugin naming convention when adding out-of-tree plugin).

cc: @shmuelk @ahg-g @elevran @kfswain

[shmuelk]

/assign shmuelk

[nirrozenbaum]

@shmuelk @ahg-g I see #1110 was merged, anything left in this issue?

[ahg-g]

I think we are good