revise env vars and secrets page

[josefaidt]

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

[timngyn]

Related issue to duplicate h3 headings: #6748

[reesscot]

@josefaidt Removing engineering label since there's already another issue for duplicate h3 headsing.