Four cross-reference links used the label "Index" instead of the
destination page's title, making them ambiguous for readers. Replaced
with the correct titles: "About Gateways and Sandboxes" and "About
Inference Routing".
Close the missing closing parenthesis on the Manage Providers link in
github-sandbox.mdx and correct four instances of "Github" to "GitHub"
across the docs.
Signed-off-by: mesutoezdil <mesudozdil@gmail.com>
Drew Newberry