docs: add docs for docs.ollama.com (#12805)

docs/context-length.mdx

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+---
+title: Context length
+---
+
+Context length is the maximum number of tokens that the model has access to in memory.  
+
+<Note>
+  The default context length in Ollama is 4096 tokens.
+</Note>
+
+Tasks which require large context like web search, agents, and coding tools should be set to at least 32000 tokens.
+
+## Setting context length
+
+Setting a larger context length will increase the amount of memory required to run a model. Ensure you have enough VRAM available to increase the context length.
+
+Cloud models are set to their maximum context length by default.
+
+### App
+
+Change the slider in the Ollama app under settings to your desired context length.
+![Context length in Ollama app](./images/ollama-settings.png)
+
+### CLI
+If editing the context length for Ollama is not possible, the context length can also be updated when serving Ollama.  
+```
+OLLAMA_CONTEXT_LENGTH=32000 ollama serve
+```
+
+### Check allocated context length and model offloading
+For best performance, use the maximum context length for a model, and avoid offloading the model to CPU. Verify the split under `PROCESSOR` using `ollama ps`.
+```
+ollama ps
+```
+```
+NAME             ID              SIZE      PROCESSOR    CONTEXT    UNTIL
+gemma3:latest    a2af6cc3eb7f    6.6 GB    100% GPU     65536      2 minutes from now
+```