docs(self-hosted): document per-deployment runtime env values, drop --only-deps

[Ryan-Amirthan]

Summary

Documents the deferred env var substitution shipped in fern-platform#14306 and fern#17578, and removes the deprecated fern-generate --only-deps runtime-generation flow from the self-hosted docs.

New Per-deployment values section on the self-hosted setup page, written around the customer-facing path: write the value as ${VAR} with substitute-env-vars and list its name in FERN_RUNTIME_ENV_VARS at build time, and generation rewrites it to a FERN_SELF_HOSTED_ENV_<NAME> placeholder that the container resolves from its own environment on each request. Also notes that url/custom-domain always resolve at build time, that Markdown/llms.txt and search results are substituted too, that a runtime value may include or omit a scheme, and that an unlisted or unset name renders as the literal placeholder. FERN_RUNTIME_ENV_VARS added to the env var table.

Removals: the ### Runtime generation section, the "vendor dependencies for runtime generation" air-gapped accordion (Option 3 renumbered to Option 2), and the commented --only-deps alternative in the remaining Dockerfile examples. Build-time RUN fern-generate guidance is unchanged. Translations still carry the old text and are left to the nightly sync.

Plus a changelog entry (merged alongside the search-metadata entry for the same date).

Testing

• pre-commit run --files <changed>: vale passed, 0 errors on the self-hosted page and changelog.
• fern check --warnings: 0 errors; the 12 warnings are pre-existing (unrelated API path conflicts, generator config MDX parse warnings, FDR unreachable).
• rg "only-deps|skip-deps|runtime generation" over fern/**/*.mdx (excluding translations): no matches.
• Preview: https://fern-preview-devin-1788221707-self-hosted-runtime-env.docs.buildwithfern.com/learn/docs/self-hosted/set-up#per-deployment-values

Link to Devin session: https://app.devin.ai/sessions/48343062b0d241d1b2319f33603aa999
Open in Devin Desktop: https://app.devin.ai/desktop/session/48343062b0d241d1b2319f33603aa999?variant=devin
Requested by: @Ryan-Amirthan

[devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment, CI, and merge conflict monitoring

[github-actions]

🌿 Preview your docs: https://fern-preview-devin-1788221707-self-hosted-runtime-env.docs.buildwithfern.com/learn

Here are the markdown pages you've updated:

• learn/docs/changelog/2026/9/1
• learn/docs/self-hosted/set-up