Add documentation for improved client assertions support

[Erwinvandervalk]

• im
• imoc
• added client assertions sample link

[Copilot]

Pull request overview

Adds and cross-links documentation for client assertions support across IdentityModel and related libraries, including guidance for DPoP retry scenarios and a runnable sample reference.

Changes:

• Added a new IdentityModel “Client Assertions” endpoint doc covering ClientAssertion and ClientAssertionFactory (including DPoP retry motivation).
• Updated OidcClient DPoP docs to note support for combining DPoP with client assertions.
• Fixed note-block formatting and added a working sample link in Access Token Management docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
astro/src/content/docs/identitymodel/endpoints/client-assertions.md	New endpoint documentation for client assertions and the new factory-based API.
astro/src/content/docs/identitymodel-oidcclient/advanced/dpop.mdx	Adds a note linking DPoP usage to client assertions documentation.
astro/src/content/docs/accesstokenmanagement/advanced/client-assertions.mdx	Fixes admonition closing and links to a complete sample implementation.

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