doc: sketch constraint example uses "Fixed", which the constraint table below it does not list

## Where

The Sketch chapter, "constraint-based sketching" section (`doc/sketch.rst`, rendered at https://cadquery.readthedocs.io/en/stable/sketch.html). Observed on the `stable` build (v2.8.0, commit fb4c6d4).

## The inconsistency

The worked example constrains with `"Fixed"`:

```python
result = (
cq.Sketch()
.segment((0, 0), (0, 3.0), "s1")
.arc((0.0, 3.0), (1.5, 1.5), (0.0, 0.0), "a1")
.constrain("s1", "Fixed", None)
.constrain("s1", "a1", "Coincident", None)
.constrain("a1", "s1", "Coincident", None)
.constrain("s1", "a1", "Angle", 45)
.solve()
.assemble()
)
```

Two paragraphs below, the "Following constraints are implemented" table lists `FixedPoint`, `Coincident`, `Angle`, `Length`, `Distance`, `Radius`, `Orientation`, `ArcAngle` — **`Fixed` is not in the table**, and `FixedPoint`'s documented argument (`None` for arc center or a 0..1 float) is not what the example passes.

A reader can't tell whether `Fixed` is an undocumented alias, a distinct constraint the table omits, or something deprecated that the example should stop using.

## Measured on 2.8.0 (this is how the inconsistency was found)

Running the example verbatim on cadquery 2.8.0 / Python 3.12: it **executes successfully** — `solve()` converges and `assemble()` returns a usable face (extrudes to a valid solid). So the example's `"Fixed"` form is real, working API that the table doesn't document.

## Suggestion

Either add a `Fixed` row to the constraint table (arity 1, argument `None`, "entity is fully fixed"), or — if `FixedPoint` is the intended public spelling — update the example to use it with its documented argument form. Happy to PR whichever direction maintainers prefer.

Open doc/sketch.rst and inspect the "constraint-based sketching" section, its worked example, and the following constraint table. Confirm the documented spelling and argument form against the example, then make the example and table consistent in the direction maintainers choose. Verify the rendered Sketch documentation and example wording afterward.

Written by the indexing model from the issue text.