Adding the missing Internal Articles API docs (#292)

* adding back the internal article docs

* fix: resolve UpdateInternalArticleRequest duplicate declaration

Override the PUT /internal_articles/{id} endpoint in unstable API to use
a unique request name (UpdateInternalArticleBodyRequest) to avoid
conflicts with auto-generated type names.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

* fix: rename update_internal_article_request schema to avoid duplicate declaration

Change the x-fern-type-name for update_internal_article_request schema to
UpdateInternalArticleRequestBody to prevent conflicts with auto-generated
request types in the unstable API.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

* fix: mark update_internal_article_request as non-nullable

The schema was incorrectly marked as nullable, causing Java SDK generation
to fail with "cannot find symbol: method isPresent()" errors.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

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Co-authored-by: mailson <[email protected]>
Co-authored-by: Claude <[email protected]>

Author: 3 people