Fix wiki main page not displaying custom layout

[Joehuu]

• Regressed in Fix wiki link path inconsistencies #26378

Before	After

[bdach]

Can you explain what is happening here? What's with the casing changes? How did you decide which casing is "right"? Why does one place do .ToLower() arbitrarily? Why do some places hardcode Main page and some use a const?

[Joehuu]

What's with the casing changes?

It was to bring in line the rest of the casing consistency to tests and fix TestSceneWikiOverlay showing index > Main Page (with an upper P).

How did you decide which casing is "right"?

Wiki pages/paths are sentence-cased.

Why does one place do .ToLower() arbitrarily?

Custom page layout strings are the lowercased path string, hopefully. There are two layouts (the only custom one being main_page and the other one being markdown_page that is used in every other page:
https://github.com/ppy/osu-web/blob/b6ed9bebe3ef1ad0231a16bbe8a37c58f580eecd/app/Models/Wiki/Page.php#L38-L41

Why do some places hardcode Main page and some use a const?

Thought it was fine that tests have hardcoded things. The tests just copy the API json output.