Adds documentation about the release procedure and breaking changes.

Fix all the dead links and add two new lines before each new header according to some form of markdown best practices.

Some links still remain broken after flame-engine#1663, as reported by linkcheck. This PR fixes all of them, except these 5:

https://examples.flame-engine.org/#/Collision%20Detection_Circles

https://examples.flame-engine.org/#/Collision%20Detection_Shapes%20without%20components

https://examples.flame-engine.org/#/Collision%20Detection_Multiple%20shapes

https://examples.flame-engine.org/#/Rendering_Isometric%20Tile%20Map

https://examples.flame-engine.org/#/Controls_Joystick

…ne#1657)

As described flame-engine#1546, It would be more convenient and can reduce the game widget updates if adding or removing overlays at once.

…ngine#1659)

Make sure that subscription to GameStateChange events is properly updated when the game: property of a GameWidget changes.

changeParent should be allowed even when the current parent is null.

Fix the last remaining broken example links.

This was a planned move once we'd switch to Dart 2.15; and we're already on 2.16 since flame-engine#1617.

…-engine#1675)

We were planning to make this change once we upgrade min Dart version to 2.16; which we finally did in flame-engine#1617.

In some places in code we used patterns like fold(0, (a, b) => a + b), which can be replaced with a simple .sum from the Iterable extension.

…lame-engine#1673)

These methods can be simplified using the extension Iterable.firstOrNull getter.