docs(fern): add SEO meta descriptions + headlines to nightly docs (#1589)

[lbliii]

What

Fills in per-page SEO description frontmatter for the main/latest (nightly) docs and adds headline SEO-title overrides for generic titles. Scoped to fern/versions/latest/pages only.

Closes #1589.

Why

Investigating the live site (docs.nvidia.com/nemo/gym) showed the issue's suspected problems are already handled by Fern's defaults:

• ✅ sitemap.xml published (108 URLs) and listed in robots.txt
• ✅ Googlebot/Bingbot allowed; only tracking-param URLs disallowed
• ✅ Cross-version duplicate content collapsed via auto rel=canonical (/main/… and /v0.3.0/… both canonical to the un-prefixed URL)
• ✅ Old version /v0.2.1/* already noindex

The actual gap was per-page metadata: 57 of 71 indexed pages had description: "". With no meta description, search engines fall back to generic body snippets, so deep pages have weak SERP relevance and the landing page outranks them for specific queries — exactly the user-reported "I only land on the main page."

Changes

• 63 pages updated under fern/versions/latest/pages (the indexed default "Main"/nightly version).
• Descriptions: 74–165 chars, unique across all pages, each adversarially verified against the page body (no fabricated features, flags, or numbers).
• 28 headline overrides give query-matching <title> tags to generic titles (Overview, FAQ, Setup, …) without changing the visible H1.
• The v0.3.0 mirror and noindex'd v0.2.1 are intentionally not touched.

Verification

• fern check → 0 errors.
• Diff is frontmatter-only (only description/headline lines added; only empty/thin description lines removed); no body edits.
• Independent re-scan: 0 empty, 0 thin (<70 chars), 0 duplicate descriptions.

Follow-ups (not in this PR)

• Add a site-level metadata: block in docs.yml (og:image, og:site_name, twitter:site) for richer social-share cards.
• Confirm the docs property is verified in Google Search Console and the Fern sitemap is submitted.

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[cwing-nvidia]

Suggested change

	description: "Browse NeMo Gym release notes by version, from v0.3.0 back to v0.1.1, covering new environments, agent harnesses, integrations, bug fixes, and deprecations."
	description: "Browse NeMo Gym release notes by version, covering new environments, agent harnesses, integrations, bug fixes, and deprecations."

[cwing-nvidia]

Suggested change

	description: "The NeMo Gym Agent server orchestrates rollouts: it runs the agent loop, calls the Model server, routes tool calls to Resources, and collects the final reward."
	description: "The NeMo Gym Agent server houses the agent harness, which wraps a model with an agent loop."

[sephmard]

Great work on the metadata sweep — the bulk of this PR (55+ pages) is accurate and ready. A rebase is needed before merge due to conflicts and stale CLI references from PRs that landed after this was opened.

Merge conflicts — 3 files need manual resolution:

• resources-server/index.mdx — deleted by #1781. The proposed description ("The NeMo Gym Resources server defines an agent's task, tools, and per-session state...") should move to build-verifiers/index.mdx, which currently has no description.
• environment-tutorials/verification-patterns/index.mdx — deleted by #1771. The proposed description ("Choose a verification pattern to score agent behavior...") should move to build-verifiers/verification-patterns/index.mdx, which also has no description.
• reference/cli-commands.mdx — main already has description: "Reference for the unified gym CLI: command groups, flags, and migration from the legacy ng_* commands." from the CLI rewrite. The proposed description ("Reference for every NeMo Gym CLI command (ng_ short and nemo_gym_ full forms)...") is also stale — ng_* is now a deprecated alias, not the primary form.

Stale CLI references in descriptions — 4 files will merge cleanly but put incorrect text in the docs:

File	Current description says	Should be
data/index.mdx	ng_prepare_data	gym dataset collate
data/download-huggingface.mdx	ng_download_dataset_from_hf	gym dataset download
data/prepare-validate.mdx	ng_prepare_data	gym dataset collate
infrastructure/engineering-notes/system-design.mdx	"starts up on ng_run"	"starts up on gym env start"

Happy to approve once rebased and those 7 files are updated.

[sephmard]

Great work on the metadata sweep — the bulk of this PR (55+ pages) is accurate and ready. A rebase is needed before merge due to conflicts and stale CLI references from PRs that landed after this was opened.

Merge conflicts — 3 files need manual resolution:

• resources-server/index.mdx — deleted by #1781. The proposed description ("The NeMo Gym Resources server defines an agent's task, tools, and per-session state...") should move to build-verifiers/index.mdx, which currently has no description.
• environment-tutorials/verification-patterns/index.mdx — deleted by #1771. The proposed description ("Choose a verification pattern to score agent behavior...") should move to build-verifiers/verification-patterns/index.mdx, which also has no description.
• reference/cli-commands.mdx — main already has description: "Reference for the unified gym CLI: command groups, flags, and migration from the legacy ng_* commands." from the CLI rewrite. The proposed description ("Reference for every NeMo Gym CLI command (ng_ short and nemo_gym_ full forms)...") is also stale — ng_* is now a deprecated alias, not the primary form.

Stale CLI references in descriptions — 4 files will merge cleanly but put incorrect text in the docs:

File	Current description says	Should be
data/index.mdx	ng_prepare_data	gym dataset collate
data/download-huggingface.mdx	ng_download_dataset_from_hf	gym dataset download
data/prepare-validate.mdx	ng_prepare_data	gym dataset collate
infrastructure/engineering-notes/system-design.mdx	"starts up on ng_run"	"starts up on gym env start"

Happy to approve once rebased and those 7 files are updated.