Updates the SSO docs to also cater to classic/Oauth based flow.

[lavin-ds]

Description

Updates the SSO docs to also cater to classic/Oauth based flow.

Reasons

https://circleci.atlassian.net/browse/CIAMP-1416

Content Checklist

Please follow our style when contributing to CircleCI docs. Our style guide is here: https://circleci.com/docs/style/style-guide-overview.

Please take a moment to check through the following items when submitting your PR (this is just a guide so will not be relevant for all PRs) 😸:

• Break up walls of text by adding paragraph breaks.
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• Keep the title between 20 and 70 characters.
• Consider whether the content would benefit from more subsections (h2-h6 headings) to make it easier to consume.
• Check all headings h1-h6 are in sentence case (only first letter is capitalized).
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