SDL3/MigrationGuide: Added.

[SDLWikiBot]

This edit was made by @Perry-chouteau.

Live page is here: https://wiki.libsdl.org/SDL3/MigrationGuide

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[Perry-chouteau]

I edit some error on the website and it create a lot of pull_request sorry

[icculus]

No worries! One thing I want to do (but haven't started) is have additional edits add to the same pull request instead of generating a new one each time, but we understand the multiple-pull-request thing happens from time to time right now.

We are still deciding what to do about a migration guide, though. The changes from SDL 1.2 to SDL2 involved a lot more work on changing how the app should approach problems, whereas an SDL2 to SDL3 guide would be more about adjustments to APIs. I think you have the start of the right approach here: build your app, see if there's an error about SDL_x_renamed_to_SDL_y and work forward from there. Maybe the 2-to-3 migration guide should be about the techniques one should use to migrate instead of the specific technical details!