Add tutorial & about page

[x64x2]

Help section detailing how to play the game is essential in Sudoku for people looking to play but don't know how

I think you should add it in the next update

[TheSunCat]

Thanks for the suggestion! How do you envision this section looking/working? A first-boot "onboarding" screen could work pretty well, such as the one in DAVx^5, but I'd like to get feedback before implementing it.
I'm open to adding features, though I don't want to make the codebase too large.

[TheSunCat]

I've finished fully decoupling the game board as a widget from the gameplay screen, and on my local machine I have a functional onboarding screen with each page having an interactive Sudoku board.

However, I'm unsure as to how to go about actually doing the tutorial. I would appreciate any suggestions on how I could approach this, to teach the game in 3-4 interactive screens.

[TheSunCat]

I've added a simple "game rules" tutorial that looks like this:

If you input the wrong number, it gets highlighted red, so it should give the player a good example of how the game is played.
Any suggestions welcome, but if not (with some more polish) this will be in the next release.

[TheSunCat]

See changes in de69dd8 for my implementation this feature. Feedback very welcome!

Ping: @s0mt0chukwu @Wjxfi @bloodyeyesblackblood