docs: add fern versioning

[miyoungc]

Description

Use Fern's Git-ref version publishing to add immutable historical documentation versions without copying release content into the current working tree.

• Keep Latest backed by the current docs/index.yml.
• Publish v0.23.0 from the stable v0.23.0 Git tag.
• Publish deprecated v0.22.0 from commit fb73f41b0fa2ef6c50c90d76f8b20533e4c9a51e. The v0.22.0 release tag predates the Fern migration, while this curated post-migration commit contains the required Fern configuration, navigation, and OpenAPI files.
• Pin Fern CLI 5.91.0, which supports ref-backed documentation versions.
• Document how to choose an immutable tag or commit SHA and clarify that fern docs dev renders working-tree versions only.

Using immutable refs keeps historical versions stable across republishes and avoids a mutable backfill branch. Areas for careful review are whether the selected v0.22.0 commit represents the intended snapshot and the stable/deprecated availability labels.

Related Issue(s)

Follow-up to #2262. No triaged issue assigned to the PR author is currently linked.

Verification

• make docs-fern — passed with 0 errors and 4 existing removed-SDK-page warnings.
• uv run --locked pre-commit run --files docs/LIVE_DOCS.mdx docs/README.mdx fern/docs.yml fern/fern.config.json — passed.
• git diff --check — passed.
• Confirmed v0.23.0 and commit fb73f41b0fa2ef6c50c90d76f8b20533e4c9a51e contain the required Fern configuration and navigation files.
• Confirmed the v0.22.0 snapshot commit is reachable from origin/develop.

A published Fern preview of the ref-backed versions was not run. GitHub CI is pending.

AI Assistance

• No AI tools were used.
• AI tools were used; a human reviewed and can explain every change (tool: OpenAI Codex; used for implementation, documentation, validation, and PR description drafting).

Checklist

• I've read the CONTRIBUTING guidelines.
• This PR links to a triaged issue assigned to me.
• My PR title follows the project commit convention.
• I've updated the documentation if applicable.
• I've added tests if applicable.
• I've noted any verification beyond CI and any checks I couldn't run.
• I did not update generated changelog files manually.
• I addressed all CodeRabbit, Greptile, and other review comments, or replied with why no change is needed.
• @mentions of the person or team responsible for reviewing proposed changes.

[github-actions]

Staged Fern docs preview: https://nvidia-preview-pr-2287.docs.buildwithfern.com/nemo/guardrails

[codecov]

Codecov Report

❌ Patch coverage is 99.13958% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
nemoguardrails/cli/chat.py	40.00%	3 Missing ⚠️
nemoguardrails/actions/action_dispatcher.py	98.29%	2 Missing ⚠️
nemoguardrails/cli/debugger.py	33.33%	2 Missing ⚠️
nemoguardrails/embeddings/providers/fastembed.py	0.00%	1 Missing ⚠️
nemoguardrails/embeddings/providers/nim.py	0.00%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!