Backport of identity/oidc: updates documentation for PKCE, client types, and default resources into stable-website

[hc-github-team-secure-vault-core]

Backport

This PR is auto-generated from #14636 to be assessed for backporting due to the inclusion of the label backport/website.

WARNING automatic cherry-pick of commits failed. Commits will require human attention.

The below text is copied from the body of the original PR.

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This PR adds documentation for a few features that were recently added to Vault OIDC provider:

• PKCE (identity/oidc: Adds proof key for code exchange (PKCE) support #13917)
• Client types (identity/oidc: Adds proof key for code exchange (PKCE) support #13917)
• Default resources (identity/oidc: Adds default provider, key, and allow_all assignment #14119)

Additionally, this PR reworks some of the content:

• General documentation page follows "Setup" -> "Usage" convention of other documentation
• General documentation uses default resources for a minimal working configuration
• General documentation references other HashiCorp products with OIDC auth methods that we've tested