refactor: refactor examples to use redirect instead of callback

[raynerljm]

Problem

Non-standard naming (we use callback and redirect interchangeably in our dev docs/README/SDKs, etc.) In an effort to standardize the term, this PR changes all the examples to use redirect instead of callback.

This is done together with standardizing the dev docs and the developer portal

How did you solve the problem?

For Express example:

• Change redirectUri definition in SgidClient to be /redirect instead of /callback
• Change the endpoint to /redirect from /callback

For Next.js CSR example:

• Change redirectUri definition in SgidClient to be /redirect instead of /callback
• Change the API route handler file name to redirect.ts from callback.ts

[kwajiehao]

lgtm