Improve naming and organization of some manual articles

[YuriSizov]

This PR includes several changes aimed at improving sidebar navigation or overall organization of the documentation.

• In "Platforms" HTML5/Web platform pages were moved to their own subfolder, web;
• In "3D" Global Illumination pages were moved to their own subfolder, global_illumination;
• "User Interface (UI)" lost its excessive capitalization to become "User interface (UI)";
• "Input and Output (I/O)" was renamed to "File and data I/O";
• "Inputs" was renamed to "Input handling";
• "2D" and "3D" sections had their tutorials grouped to reduce the size of the initial list, give some order, and ultimately improve navigation;
• Also some no-longer in-use images were dropped.

After these changes the top level of the "Manual" category looks like this.
And here are the 2D and 3D sections:

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All pages which changed their path were added to redirects.

[akien-mga]

Looks great! You can rebase the temporary fix away.

[YuriSizov]

You can rebase the temporary fix away.

@akien-mga Wait, but I don't think the CI will pass since the class ref hasn't been synced with this repo yet?

PS. I can do it myself, if you want. Done.

[YuriSizov]

I don't remember how we resolved this before, if at all, but naturally updating class ref first also fails the CI for that particular commit 🙃 Should be good after both changes are merged now.