Reflect the move from Gitlab to Github.

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Currently, these docs link to the archived gitlab project, rather than to the current Github ones.

For example, under the "Sia 101" page, the contributors guide links to gitlab.com/NebulousLabs/Sia/blob/master/CONTRIBUTING.md, rather than to https://github.com/SiaFoundation/siad/blob/master/CONTRIBUTING.md.

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I've started to change references, however there are three things I'm not sure what to do with:
The common api requests section.

In wallet-overview.md it links to the old gitlab page, where the latest release is 1.4.7, and I'm not sure where the current repo for sia-ui is.

Referenced here, I can't find where the current repo for nanos-app-sia is.

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I'm not sure how to properly link it, but I've created a pull request.