feat: About window; Documentation cleanups; Window sizing

[matatk]

• Implement an "About" box (this is shown on start-up until the game has been run at least once, and is accessible via the launcher's "Help" tab too).
• Include more documentation in the launcher's GUI (it's also still in the "docs" directory.
• Don't allow the launcher window to be resized.
• Cap the size of the HTMLPageDialogs (though they can be maximised/resized still).
• Update more "2020" to "2021" references.
• Rename the LDL tutorial file (the abbreviation should've been upper-case).