Implement adding a rectangle to a pdf layer

[dodomorandi]

This should improve the support to writing PDFs towards two directions:

1. The ability of stroke/fill rectangles
2. The possibility of using simple clipping operations based on rectangles

The changes should be pretty straightforward, and I also added a few changes/fixes to the doc. Still, there are a couple of points I would like to discuss:

• I added Mode and Winding specifically for the rectangle module. However, we could consider to unify those with the one available inside the line module. The Winding enum cannot actually be any different, I am a bit unsure about the Mode because I don't have a clear view about other possibilities (i.e: is there a paint mode that we would like to add in the future that makes sense for a Rect but not for a Polygon or vice versa?)
• While writing the implementation for into_stream_op, I noticed that Polygon adds a paint end even after a stroke/fill. AFAIK (reading from the PDF specs) this should be only necessary if a path-painting operator has not been used -- and strokes/fills are actually path-painting operators. I am pointing this out just because if I am right, I don't think it should be an actual problem (probably any pdf reader does not have any trouble ignoring the n operator), but if I am wrong I should change the implementation for Rect, because I only add the path end operator only in clipping mode.

Let me know what do you think, and feel free to give any suggestion I could improve the code or the doc.

Last but not least: this is a breaking change because of the new Rect::mode field (and cargo-semver-checks confirms that).

CC @paolobarbolini

[dodomorandi]

I moved WindingOrder and PolygonMode to a path module, and I also renamed PolygonMode to PaintMode. Because of this change I also took the opportunity of add some docs and some minor changes.

Just keep in mind that at this point we still miss the possibility of choosing between open and closed paint operations in polygons, but it probably makes sense to have a flag specifically in the Polygon struct (because it does not make sense for rectangles and circles) and therefore it should be handled in another PR.

Let me know if it's ok for you or you would rather to change things a bit or to split this PR in two (because of the refactor).