docs: lead with env vars in admin docs and add configuration reference (#26824) - #27806

#26824)

## What & why

Admin/setup docs lead with `coder server --flag` examples, but most
operators configure Coder through `CODER_*` environment variables
(system service, container, or Helm chart). There is no single page
mapping a setting to its env var, CLI flag, YAML key, and default, so
searching the docs for an env var name such as `CODER_PG_CONNECTION_URL`
returns nothing.

This adds a generated configuration reference and begins shifting admin
docs to lead with the environment-variable form.

## Changes

- **Generated configuration reference**
(`docs/admin/setup/configuration-reference.md`): a searchable,
per-setting list of every visible deployment option. Each option is a
heading (grouped and nested by serpent group) followed by its
description and the environment variable, CLI flag, YAML key, and
default that apply to it. Generated from `codersdk.DeploymentValues` so
it stays in sync.
- **Generator + `make gen` wiring** (`scripts/configdocgen/`): new
binary plus a Makefile target and `GEN_FILES` entry, mirroring the
existing `clidocgen` / `auditdocgen` pattern. Output is host-independent
(same env normalization as `clidocgen`).
- **Demo conversion** (`docs/admin/users/github-auth.md`): inverted to
lead with the `/etc/coder.d/coder.env` env-var form; the CLI-flag form
becomes a closing note that links to the reference. H2 slugs preserved.
- **Style guide** (`.claude/docs/DOCS_STYLE_GUIDE.md`): documents the
env-var-first convention for admin/setup docs.
- **Navigation**: manifest entry under Administration → Setup, plus a
TIP callout on the setup index.

## Risk

Docs + gen pipeline only; no runtime change. The page is regenerated by
`make gen`; the `gen` and `check-docs` CI checks pass.

## Follow-up

Several other admin pages still lead with flag walls. Recommend sweeping
them incrementally in separate PRs rather than expanding scope here.

<details>
<summary>Implementation notes (provenance, conflict resolution,
verification)</summary>

- Continues prior work by @aslilac and @bpmct from the
`kayla/docs-env-vars-first` branch. Both original commits are
cherry-picked here with authorship preserved.
- Rebased onto current `main`. Resolved two `Makefile` conflicts where
`main` had since added the `feature-stages.md` gen target at the same
locations; kept both targets (union) in `GEN_FILES`, `gen/mark-fresh`,
and the recipe block.
- The original branch's checked-in page predated recent
`codersdk.DeploymentValues` changes, so it was **regenerated** against
current `main` (adds `CODER_SCIM_USE_LEGACY`, the `Networking / Cluster`
section with `CODER_CLUSTER_HOST`, `CODER_BOUNDARY_LOG_RETENTION`, and
the AI Gateway description rename). The `gen` CI check enforces this
stays current.
- Fixed flag-link anchors for short-form flags (`--config`,
`--log-filter`): the generator derives the anchor from `FlagShorthand`
to match `clidocgen`'s heading (e.g. `#-l---log-filter`).
- `linkspector` ignores the AWS Bedrock base URL that appears as an
illustrative `<region>` placeholder in an option description, consistent
with the existing `openai.com` ignore patterns.

</details>

<details>
<summary>Configuration reference layout (2026-07-08 update)</summary>

Reworked the reference from a wide table into a nested, per-setting list
so it fits without horizontal scrolling and stops repeating the group
name in every heading:

- **List, not table.** Each option renders as a heading, its
description, and a bullet list of only the configuration methods that
apply to it (non-applicable methods are omitted instead of shown as
`-`).
- **Nested sections.** Sections nest by the serpent group hierarchy, so
`Email / Email Authentication` becomes `Email` (h2) with an `Email
authentication` (h3) subsection instead of a redundant flat title.
- **Shorter, sentence-case headings.** The redundant group prefix is
stripped from each option name and the remainder is lowercased to
sentence case, preserving acronyms and mixed-case tokens (`URL`, `TLS`,
`OAuth2`, `GitHub`) plus a small proper-noun allowlist (`Coder`,
`Terraform`, `Honeycomb`, `Anthropic`, `Bedrock`, ...). Example: `AI
Gateway Send Actor Headers` becomes `Send actor headers`.
- **Deprecated options** sort to the end of each section and lead with
an emphasized **Deprecated** marker. Headings stay clean (no
`(deprecated)` suffix) so their anchors remain stable.
- **Section intros** render from a group's `Description` when the source
defines one (e.g. DERP); no hand-maintained prose or links are
introduced.

All transformations run in pure Go at `make gen` time (no AI at
generation time). Generation is idempotent, and `markdownlint` and
`golangci-lint` both pass.

</details>

---

🤖 Opened by Coder Agents on behalf of @nickvigilante. Continues work by
@aslilac and @bpmct.

---------

Co-authored-by: Kayla (via Coder Agents) <[email protected]>
Co-authored-by: Coder Agents <[email protected]>
Co-authored-by: Ben Potter <[email protected]>
(cherry picked from commit ba4779f)