sagemathgh-37851: Fix issue 37587 regarding the Link class plot method
    
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As suspected in
sagemath#37587 (comment)
the issue can be fixed by adding the contraint for the exterior region.

In the differences of this branch just the lines below belong to the
corresponding correction of the algorithm:


```
index 9d988d8..33d0cae051 100644
--- a/src/sage/knots/link.py
+++ b/src/sage/knots/link.py
@@ -3607,36 +3607,55 @@ class Link(SageObject):
         # such that the resulting regions are in fact closed regions
         # with straight angles, and using the minimal number of bends.
         regions = sorted(self.regions(), key=len)
-        regions = regions[:-1]
         edges = list(set(flatten(pd_code)))
...
+            else:
+                # capacity of exterior region, only sink
+                capacity = len(r) + 4
...
         nregions = []
-        for r in regions:
+        for r in regions[:-1]:  # interior regions
             nregion = []
```

All other changes are intended to improve the readability of the code.
To this end I change and introduce new variable names to make the origin
of the ideas coming from the class [OrthogonalLinkDiagram](https://githu
b.com/3-
manifolds/Spherogram/blob/3062478dd69a52a16e55b967dd46dfe940af9ea7/spher
ogram_src/links/orthogonal.py#L118) of
[Spherogram](https://github.com/3-manifolds/Spherogram) more explicit.
Furthermore I make some codestyle fixes and change the sign of the
capacity such that the sink appears to be positive and thus according to
the usage in Spherogram and the literature cited there.

With this branch the knot reported in the issue is plotted like this:

```
sage: kn = Knots().from_dowker_code([30, 18, 20, 24, 22, 26, 28, 32, 2,
4, 6, 8, 12, 10, 16, 14])
sage: arcs = sorted(kn.arcs())
sage: cols = {tuple(arcs[i]): i for i in range(len(arcs))}
sage: kn.plot(color=cols)
```

### New plot

![kn_new_cols](https://github.com/sagemath/sage/assets/47305845/328ff242
-2a99-48b1-b55a-657f13db0fae)

To make comparison with other results more convenient I used different
colours for the arcs. The corresponding result for `PPL` is this:

```
sage: kn.plot(color=cols, solver='PPL')
```

### New plot with `PPL`

![kn_new_cols_ppl](https://github.com/sagemath/sage/assets/47305845/359b
2404-91a4-44bc-a949-5cc7dec9ba0f)


Without this branch the `PPL` version with colours has been this:

### Old plot with `PPL`

![kn_cols_ppl](https://github.com/sagemath/sage/assets/47305845/5bc9809c
-9aa5-4cb4-b700-7c833e5e598a)

The coloured wrong result has been this:

### Old plot

![kn_cols](https://github.com/sagemath/sage/assets/47305845/513651b5-
8aa7-4394-b5e1-0d6715e7fbe1)


Note, that this branch also changes other plot results in the
documentation. For example:

Previously:

### Old documentation

![grafik](https://github.com/sagemath/sage/assets/47305845/41ef8ad9-
cbbd-42ed-a898-3cd565bab23e)


With this branch:

### New documentation

![grafik](https://github.com/sagemath/sage/assets/47305845/4bcff688-
10e6-4a79-9823-12205d58ac75)




### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [x] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#37851
Reported by: Sebastian Oehms
Reviewer(s):