revise env vars and secrets page

**Describe the content issue**:
A clear and concise description of what the content issue is. Is there content missing or is there incorrect content?

- duplicate h3 headings are causing issues with anchors
- local env workflow at the bottom states you can use `.env.local`, however this only applies to some frontend frameworks and not the backend
- migrate from CRA-flavored env vars to Next.js or Vite as the default (or some generic var, but ideally these are InlineFilters)
- change slug to `/environment-variables-and-secrets` instead of `/secrets-and-vars` to align with Functions

**URL page where content issue is**:

https://docs.amplify.aws/react/deploy-and-host/fullstack-branching/secrets-and-vars/#local-environment