Docs: Update SSO with Auth0 guide DRAFT

[aimeeu]

Description

The Jira tickets asks to clarify the use of callback URL localhost:4649 in the guide. When I tested the guide, I noticed that the Auth0 steps are outdated.

• Rewrote Auth0 section to remove step-by-step guide. We don't duplicate how to perform an action in an external product, so I refactored to tell the user what is needed and link to Auth0 docs. Exception: Create a post-login trigger. Auth0 doesn't have docs on how to do that. (I also found Auth0 and Nomad docs content that no longer matches the Auth0 dashboard UI).
• Style guide updates

Note
The process in the original tutorial works, but I couldn't find any Auth0 documentation to support the custom action content, so I updated using the process Auth0 has documented.

This page was moved verbatim from tutorials in Nomad IA phase 1 so backport only to stable-website and 1.10.x.

Testing & Reproduction steps

1. Deploy or have access to Nomad cluster. I spun one up on AWS.
2. Create Auth0 account.
3. Follow guide instructions.

Links

Jira: CE-1001

Deploy preview https://nomad-git-ce1001oidc-hashicorp.vercel.app/nomad/docs/secure/authentication/sso-auth0

Contributor Checklist

• Changelog Entry If this PR changes user-facing behavior, please generate and add a
  changelog entry using the make cl command.
• Testing Please add tests to cover any new functionality or to demonstrate bug fixes and
  ensure regressions will be caught.
• Documentation If the change impacts user-facing functionality such as the CLI, API, UI,
  and job configuration, please update the Nomad website documentation to reflect this. Refer to
  the website README for docs guidelines. Please also consider whether the
  change requires notes within the upgrade guide.

Reviewer Checklist

• Backport Labels Please add the correct backport labels as described by the internal
  backporting document.
• Commit Type Ensure the correct merge method is selected which should be "squash and merge"
  in the majority of situations. The main exceptions are long-lived feature branches or merges where
  history should be preserved.
• Enterprise PRs If this is an enterprise only PR, please add any required changelog entry
  within the public repository.

[tunzor]

LGTM 👍