Document sso-session token provider configuration in credentials guide

[aryansk]

Refs boto/botocore#3776

Problem

The AWS IAM Identity Center section of the credentials guide only documents the inline profile form. sso_session and [sso-session] aren't mentioned anywhere, even though botocore has supported them since 1.29.10.

That's a functional gap: the AWS SDKs and Tools reference calls the sso-session form the recommended SSO token provider configuration, and the inline form the legacy non-refreshable configuration (no automated token refresh, session fixed at eight hours). The guide currently steers readers onto the legacy path without saying so.

Change

In docs/source/guide/credentials.rst:

• Lead with the recommended [sso-session] configuration, including a profile + sso-session example.
• Document each setting, including optional sso_registration_scopes.
• Keep the existing Session usage example unchanged.
• Document the inline form as the legacy non-refreshable configuration, with a migration pointer to [sso-session].

Validation

• The modified file parses cleanly with docutils; the only reported messages are the four pre-existing ones on develop (:ref: roles unknown to plain docutils at lines 30/110/270 and a short title underline at line 351) — none in the changed region.
• All three INI examples parse with configparser.
• Diff is docs-only (+26/-3), no code paths touched.