Add SDK repo link(s) in SDK docs

We should make sure there are appropriate links to the SDK repository within the SDK docs. Since the docs used to live "in" the repo itself on Gitlab this previously wasn't a problem. There's currently a "Edit on Gitlab" link at the top of each page, but the expectation may be that the link is just a docs-specific edit. It may not be clear or intuitive that the link also take the user to the repo for the SDK itself (not just the docs).

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From slack: https://meltano.slack.com/archives/C01PKLU5D1R/p1623681403081500

@tayloramurphy mentioned:

@AJ Steers Unless I’m missing it, I’m not seeing a link to the actual repository on https://sdk.meltano.com/en/latest/ except for the “Edit on GitLab” link. It might be good to have a direct link to the repo on the homepage