docs: Mods testing, and relax dependency to Thunderstore

[Alystrasz]

(not a mod verification request)

Updates the documentation in the following aspects:

• Exports docs to a dedicated directory;
• Splits docs between different files, sorted by thematics;
• Removes Thunderstore dependency mentions;
• Adds a documentation page on how to check whether your mod submission is valid (without having to setup a server).

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Closes #61 and #62.