docs(developers.md): update the SDK & API doc link
docs(developers.md): update the SDK & API doc link

https://platform.bitgo.com/ is no longer reachable. From https://www.bitgo.com/, the public Wallet
SDK & API link points to this url. The updates for DEVELOPERS.md and the link in the About section
in the repo homepage will be helpful for new developers.

BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS,BitGo/BitGoJS