docs: add server URL templating documentation to SDK design section

[iamnamananand996]

docs: add server URL templating page to SDK design section

Summary

Adds a new documentation page explaining the server URL templating feature under the "SDK design" section of the SDKs product docs. The page covers:

• Configuring server variables in OpenAPI (servers block with variables, x-fern-default-url, x-server-name)
• Multiple base URLs (endpoint-level servers with x-fern-server-name)
• Generated SDK behavior with tabbed code examples for Python and Java
• Equivalent Fern Definition config (api.yml with url-templates, default-urls, variables)
• Language support table (Python and Java only)

Content was derived from reading the fern-api/fern repo: the OpenAPI test fixture at test-definitions/fern/apis/server-url-templating/openapi.yml, the IR schema types (ServerVariableSchema, SingleBaseUrlEnvironmentSchema, MultipleBaseUrlsEnvironmentSchema), seed-generated SDK outputs (seed/ts-sdk/server-url-templating/), and the v63-to-v62 IR migration.

Updates since last revision

• Fixed vale lint warning: changed "they are" → "they're" in the intro paragraph.
• Narrowed language support to Python and Java only per reviewer feedback. Removed TypeScript and Go code example tabs and updated the language support table accordingly.
• Verified page renders correctly via local fern docs dev (tabs, tables, code highlighting, <Icon> components all display as expected).

View original video (rec-4b70dc4e1858470bbf0f8f96382f415e-edited.mp4)

Review & Testing Checklist for Human

• Fern Definition section accuracy (medium risk): The api.yml example with url-templates, default-urls, and variables was derived from internal schema types (MultipleBaseUrlsEnvironmentSchema). Confirm this is a valid user-facing config format and not just an internal representation populated during OpenAPI import.
• x-server-name vs x-fern-server-name: The source OpenAPI fixture uses x-server-name at the top level and x-fern-server-name at the endpoint level. Verify the docs correctly explain when to use each.
• Python/Java code examples: These were written to match the general pattern but were not verified against actual seed-generated output for those languages. Spot-check against seed/python-sdk/server-url-templating/ and seed/java-sdk/server-url-templating/.
• Preview the page: Verify the preview deployment renders correctly end-to-end with the updated Python/Java-only content.

Notes

• Requested by: @iamnamananand996
• Link to Devin run

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[github-actions]

🚫 [vale] _{reported by reviewdog 🐶}
[Microsoft.Contractions] Use 'they're' instead of 'they are'.

[github-actions]

🌿 Preview your docs: https://fern-preview-8881a5fe-e1e0-4f8a-811a-32fc933f762a.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/sdks/deep-dives/server-url-templating

[github-actions]

⚠️ [vale] _{reported by reviewdog 🐶}
[FernStyles.Current] Avoid time-relative terms like 'currently' that become outdated