Reference: add landing page; fix stale /foundation/ card link

Reference section previously had an empty _index.md with no landing
content and no manualLink, so /reference/ rendered an empty page and
the section header in the sidebar had nothing to click.

Changes:

- New overview.md with linkTitle "Overview", title "Reference", and
  a table of the section's nine subsections with one-line "use it
  for" blurbs (APIs, Components, Services, SDKs, viam-server,
  viam-agent, Triggers, Device setup, Glossary).
- _index.md stubbed: kept all the legacy aliases, added manualLink
  to /reference/overview/.

Scope expansion, flagged:

- reference/viam-server.md (merged in PR viamrobotics#4971 before the
  foundation → set-up-a-machine rename in viamrobotics#4972) had a card linking
  to /foundation/, which is no longer a canonical URL. Hugo's card
  shortcode validates canonical URLs, not aliases, so the build
  errors with "Card has a bad link: /foundation". Updated the link
  to /set-up-a-machine/. This is a gap surfaced during verification
  while I was running make build-prod for the landing page, not
  anything I caused.

Verified locally: make build-prod clean; /reference/,
/reference/overview/, all nine subsection URLs, plus
/set-up-a-machine/ all return 200.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: shannonbradshaw

docs/reference/_index.md

@@ -7,6 +7,7 @@ type: "docs"
 no_list: true
 noedit: true
 open_on_desktop: true
+manualLink: "/reference/overview/"
 description: "Technical reference documentation for the Viam platform: APIs, SDKs, components, services, and more."
 aliases:
   - /reference/architecture/

docs/reference/overview.md

@@ -0,0 +1,23 @@
+---
+linkTitle: "Overview"
+title: "Reference"
+weight: 1
+layout: "docs"
+type: "docs"
+description: "Technical reference for the Viam platform: APIs, SDKs, built-in components and services, runtime behavior, supported hardware, and term definitions."
+date: "2026-04-18"
+---
+
+Technical reference for the Viam platform. Use this section when you need authoritative details on APIs, built-in models, SDKs, runtime behavior, or supported hardware.
+
+| Section                                  | Use it for                                                                        |
+| ---------------------------------------- | --------------------------------------------------------------------------------- |
+| [APIs](/reference/apis/)                 | Method reference for component, service, robot, and app APIs                      |
+| [Components](/reference/components/)     | Per-model reference for built-in components: attributes, behavior, caveats        |
+| [Services](/reference/services/)         | Per-model reference for built-in services: attributes, behavior, caveats          |
+| [SDKs](/reference/sdks/)                 | Python, Go, TypeScript, Flutter, and C++ SDK reference                            |
+| [viam-server](/reference/viam-server/)   | viam-server runtime: lifecycle, configuration, logging, CLI options               |
+| [viam-agent](/reference/viam-agent/)     | viam-agent reference: subsystems, version control, system configuration           |
+| [Triggers](/reference/triggers/)         | Trigger configuration reference                                                   |
+| [Device setup](/reference/device-setup/) | Hardware-specific setup for supported single-board computers and microcontrollers |
+| [Glossary](/reference/glossary/)         | Term definitions for the Viam platform                                            |

docs/reference/viam-server.md

@@ -476,5 +476,5 @@ Note that this method may not work for versions that are too old, as Homebrew do
 {{< cards >}}
 {{% card link="/reference/apis/" %}}
 {{% card link="/hardware/configure-hardware/" %}}
-{{% card link="/foundation/" %}}
+{{% card link="/set-up-a-machine/" %}}
 {{< /cards >}}