docs: getting_started.md and for_spectre_users.md incorrectly claim DC sweep is unavailable

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Problem

Two docs pages claim that DC sweep analysis is not yet available, but engine.run_dc_sweep() is fully implemented.

docs/getting_started.md (line 159)

The Known Limitations section says:

No DC sweep analysis yet (only DC operating point as part of other analyses)

docs/for_spectre_users.md (line 12)

The Analysis Types table has:

| .dc | .dc | engine.run_dc_sweep() | Coming soon |

Reality

run_dc_sweep() is fully implemented in vajax/analysis/engine.py (line 1394) with signature:

def run_dc_sweep(
    self,
    source: str,
    start: float,
    stop: float,
    step: Optional[float] = None,
    points: Optional[int] = None,
) -> DCSweepResult

DCSweepResult is also exported from vajax.analysis.

Fix needed

1. Remove "No DC sweep analysis yet" from the Known Limitations in getting_started.md and add run_dc_sweep() to the Available Analysis Types table.
2. Update for_spectre_users.md to change run_dc_sweep() status from "Coming soon" to "Available".
3. Add run_dc_sweep() documentation to api_reference.md (currently undocumented there).