docs: Document GPU auto-fit mode limitations and trade-offs (closes #8562) (#8954)

* docs: Add documentation about GPU auto-fit mode limitations (closes #8562)

- Document the default gpu_layers behavior (9999999) that disables auto-fit
- Explain the trade-off between auto-fit and VRAM threshold unloading
- Add recommendations for users who want to enable gpu_layers: -1
- Note known issues with tensor_buft_override buffer errors
- Link to issue #8562 for future improvements

Signed-off-by: team-coding-agent-1 <team-coding-agent-1@localai.dev>

* Apply suggestion from @mudler

Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

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Signed-off-by: team-coding-agent-1 <team-coding-agent-1@localai.dev>
Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>
Co-authored-by: team-coding-agent-1 <team-coding-agent-1@localai.dev>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>