Prominent links to Gateway and SIWF docs in READMEs

[shannonwells]

Make the main READMEs for Gateway, Social App Template link to the branded docs at https://projectlibertylabs.github.io/gateway/ and /siwf.

Even though there are links from the repo, a dev may have the repo checked out and may not see it. If the link is already in the README, make sure it's in an obvious place and marked clearly, because there are some tutorials, guides, explanations, and reference material, and in the case of SIWF there's a payload generator.