[v18] Refactor SSO docs to improve readability

[ptgott]

Backports #61492

The SSO documentation currently consists of:

• One long conceptual guide
• IdP-specific how-to guides

The conceptual guide holds a collection of topics with no unifying theme besides Teleport's SSO support, making it difficult to discover topics for readers who have not made their way through the entire guide. This change aims to improve the discoverability of SSO topics by:

• Splitting the conceptual guide into multiple pages
• Moving IdP-specific guides into their own subsection
• Moving the Login Rules docs, which only make sense if a user is setting up SSO, into the SSO section of the docs.

To split the main conceptual guide, this change:

• Turns the SSO for MFA section into a how-to guide. This mostly the structure of a how-to guide, but we couldn't use this structure because this text was a section of the conceptual SSO guide. By splitting the SSO guide, we can make this a how-to guide.
• Move the "Changing callback address" section into a partial that we can include in the IdP-specific how-to guides, since the level of detail within this section is consistent with those guides.
• Move "Working with an external identity". This belongs in Role Templates, and has the wrong level of detail for an overview of SSO. To get this to work, add H3s to the interpolation section of the Role Templates guide.
• Move all content re: configuring specific authentication connector fields into the section index page for the IdP-specific guides.
• Remove the Troubleshooting section. This text is repeated using a partial in every IdP-specific guide, and is the wrong level of detail for a general overview.
• Move the Login Rules example to where we include other Login Rule examples, in the Login Rules guide.
• Remove the tabbed examples of authentication connectors from the SSO index page. There is not nearly enough context on this page to help a reader make use of these examples. Instead, refer the reader to the IdP-specific how-to guides.
• Move "multiple SSO provider" discussion to its own page.

[github-actions]

Amplify deployment status

Branch	Commit	Job ID	Status	Preview	Updated (UTC)
paul.gottschling/61492-bp-18	ac3ba1a	6	✅SUCCEED	paul-gottschling-61492-bp-18	2026-01-05 15:22:26