docs: add citations guide (#2267)

* docs: add citations guide

Add docs/features/citations.md covering the experimental citations
surface: enabling `enableCitations` on session create/resume in all six
SDKs, reading the `citations` payload from `assistant.message` events,
the payload/type reference, how to supply citable material via document
attachments or tool `citableSources`, and current limitations.

Link the new guide from the features index.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 0c184f59-0104-4a89-a350-4096597a89b3

* docs: address review feedback on citations guide

* Fix UTF-16 span offset handling. Python slices by code point and Go
  and Rust strings are UTF-8, so all three examples now convert the
  content to UTF-16 code units before slicing. Corrected the
  accompanying tip, which wrongly implied only non-BMP characters were
  affected in Go and Rust.
* Replace the Rust example's raw JSON dump with a real span-to-source
  walk using typed_data::<AssistantMessageData>(), matching the other
  languages.
* Fall back through url, path, and id when a CitationSource has no
  title, in the Java, .NET, Go, Python, and TypeScript examples.
* Narrow the image-input.md link descriptions, since that guide covers
  image attachments rather than document support.
* Drop the "(experimental)" suffix from the features index entry; the
  guide body still carries the experimental warning.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 0c184f59-0104-4a89-a350-4096597a89b3

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 0c184f59-0104-4a89-a350-4096597a89b3

Author: patniko

docs/features/README.md

@@ -16,6 +16,7 @@ These guides cover the capabilities you can add to your Copilot SDK application.
 | [Skills](./skills.md) | Load reusable prompt modules from directories |
 | [Plugin Directories](./plugin-directories.md) | Bundle skills, hooks, MCP servers, and agents as a single loadable plugin |
 | [Session limits](./session-limits.md) | Set an AI Credits budget for a session and observe budget events |
+| [Citations](./citations.md) | Link assistant responses back to their supporting sources |
 | [Image Input](./image-input.md) | Send images to sessions as attachments |
 | [Streaming Events](./streaming-events.md) | Subscribe to real-time session events (40+ event types) |
 | [Usage and Billing](./usage-and-billing.md) | Read token counts, context-window utilization, AI credit cost, and account quota |