Add Steam Manuals scraping

[soopercool101]

Though few games use this, it would be very nice to have scraping for Steam manuals similar to the implementation for Origin. Manuals can be viewed and accessed via SteamDB (see here for an example of a game with one) so I’d imagine they’re accessible via the API. If not, they appear to always be accessible if they exist via a redirect from https://store.steampowered.com/manual/<appid>

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[soopercool101]

As an addendum, it appears the redirect method is in fact the better way to acquire the manuals, rather than relying on the GameManualURL.

Call of Juarez’s manual link in both SteamDB and Steam itself is broken, but the manual can in fact be accessed properly via https://store.steampowered.com/manual/3020/. The same applies to Psychonauts and its manual.

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