Do better organization by using subfolders

[Hans5958]

I saw that the docs is not that tidy by having all the pages on the single page. With this PR I propose to use subfolders so the pages can be organized better. I grouped it based on the parents, though it would be better if it can be grouped better.

Also, I implemented redirects to avoid breaking links, though I haven't use it at the time of writing since I don't want to move things a lot initially, as well as some changes to not depend on the parent frontmatter for the generated directory.

I'm putting this as a draft since I want to gauge interest of this before moving on further.

[Hans5958]

If this gets the greenlight, I will update more page names (e.g. touhoudb/touhoudb---page-name to touhoudb/page-name, japanese/artist-verification-japanese to japanese/artist-verification, etc). For now, I'll just do the pinned/ pages.