fix docs for window_size

[benjajaja]

1. Fix external link formatting.
2. Be specific about all fields possibly being zero, and the pixel fields being documented for linux as "unused".
3. (I am not sure about TIOCGWINSZ bing posix).
4. Specify that window_size() always returns an error on windows.
5. Put the detailed docs in WindowSize.

@markus-bauer this addresses some points of your comment in #790 after it was merged. What do you suggest regarding windows not being supported? I have tried everything in that regard:

1. Getting the console font size, but it is in "logical units", so it would need the current DPI or something like that to get to the font pixel size, to ultimately get the window size by multiplying with (columns,rows). I doubt it would be precise or reliable.
2. All of the windows console API only deals in (columns,rows).
3. Using GetClientRect, which I almost thought would work, but even that gave me bogus values. Also seems a bit far off the track for a terminal library.

Perhaps this was a mistake, and window_size() should be removed from crossterm, after all it's not cross-platform now. Users who need the window size, on unix, can then just use some tty-ioctl lib, e.g. rustix.

My motivation was to make a library that can render images in the TUI apps, supporting sixels, kitty-graphics, or even ascii as fallback. It's necessary to know a character "cell" size in pixels to correctly up- or downscale an image into a given (columns,rows) area.

[markus-bauer]

It's cool of you to spend additional time on this. The new documentation is better and links seem to work now.

Since @TimonPost merged it, I assume he's okay with adding it. I already said in the old PR, that there is probably no common solution and if you want it for unix, it's trivial to use rustix.
But it doesn't matter as long as the implementation and documentations is correct.