docs: document how fern generate picks the SDK version

[devin-ai-integration]

Summary

Users repeatedly ask how the SDK version number is chosen by fern generate, and teams on github: mode: pull-request are surprised that re-running generation on an unmerged branch doesn't keep incrementing. Nothing in the public docs covered cloud version selection: the only versioning page is the self-hosted one (--version AUTO / fern ir + fern diff), and the CLI reference only documented explicit --version X.Y.Z.

Adds a ## Version numbers section to How SDKs work covering:

• the three ways a version is determined (explicit, computed, AUTO) in a table
• the three version sources (registry, GitHub release tag, sdkVersion in .fern/metadata.json) with the highest-wins + patch-bump rule
• idempotence on an unmerged branch, and the pull-request/push + one explicit-version release run pattern
• the prerelease-line caveat and explicit-version promotion

The CLI --version flag docs get a one-line pointer to that section. --version AUTO details stay on the existing self-hosted versioning page; the new subsection links there instead of restating the setup.

Checks: pre-commit run --files <changed> (0 errors; remaining Vale warnings are pre-existing lines in commands.mdx) and fern check --warnings (0 errors).

Link to Devin session: https://app.devin.ai/sessions/dcbfc8b012a24e48ae8f64b0685dcfa8

[devin-ai-integration]

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[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1785769852-sdk-version-numbers.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/cli-api-reference/cli-reference/commands
• learn/sdks/overview/how-it-works