[MNT]: Confusing edgecolor behavior for Patch and Collection

Nobody has claimed this yet.

Noted while reviewing documentation on the current behavoir in #29252.

### Summary

Patches and Collections do not draw edges by default. This was introduced in #5596 for matplotlib 2.0. It seems that this was first achieved by a linewidth 0, but it this was changed to using an edgecolor of "none" while maintaining a finite linewidth.

As a consequence `rcParams["patch.edgecolor"]` does not have an effect by default. There are, however two cases when `rcParams["patch.edgecolor"]` becomes relevant:
1. If facecolor is "none", Patch and Collection take `rcParams["patch.edgecolor"]` as edgecolor.
I assume, this is done to prevent the artist being completely invisible (no facecolor and no edgecolor).
2. If `rcParams["patch.force_edgecolor"]` is set to true, `rcParams["patch.edgecolor"]` is used instead of "none" as the

This is overall very confusing.

Ping @efiring, who introduced `rcParams["patch.force_edgecolor"]` in #6904. Can you comment on the background and whether the proposal below is a reasonable alternative?

### Proposed fix

Can we switch to the following logic: `patch.edgecolor` is generally used as edgecolor and now defaults to "none" (to achieve the same behavior as before). To handle case 1. (prevent invisible artists) introduce a new `rcParams["patch.edgecolor_fallback"]` that has a special color only for that case.

### Needed changes to default rcParams
matplotlibrc before:
```
#patch.edgecolor: black
#patch.force_edgecolor: False
```
matplotlibrc after
```
#patch.edgecolor: "none" # default edgecolor for Patch and Collection
#patch.edgecolor_fallback: "black" # edgecolor to be used if facecolor is "none" to prevent completely invisible artists
```

### Needed changes to styles
Existing styles would migrate

- If `patch.force_edgecolor` was False:
from:
```
patch.edgecolor: COLOR
#patch.force_edgecolor: False
```
to
```
#patch.edgecolor: "none"
patch.edgecolor_fallback: COLOR
```
- If `patch.force_edgecolor` was True:
from:
```
patch.edgecolor: COLOR
patch.force_edgecolor: True
```
to
```
patch.edgecolor: COLOR
patch.edgecolor_fallback: COLOR
```

### Transition path
- `patch.force_edgecolor` gets deprecated and raises a warning when used on rcParams.
- styles that don't use any of `patch.edgecolor` or `patch.force_edgecolor` do not have to do anything.
- styles that have set `patch.edgecolor` but not `patch.force_edgecolor` are a very edge case. They either have overlooked that the default `force_edgecolor=False` makes the setting ineffective, or they have intentionally only configured the somewhat arcane edge color fallback for non-filled artists. In the former case, they get "auto-fixed" through the new behavior. I'd argue that we can affort to not runtime-warn on the latter case (but if really wanted, one could add a check at least for loading stylesheets).

No source file or test is named. Start by tracing the Patch and Collection edgecolor handling and the rcParams, matplotlibrc, and style settings described in the proposal; done means the proposed defaults, fallback behavior, force_edgecolor deprecation, and existing style migrations are consistently validated.

Written by the indexing model from the issue text.