docs: refresh index landing page for v0.1

Drop the unreleased Docker/GHCR reference (scoped for v0.3), drop the
'not yet usable' status, and align the version tag with v0.1. Add a
brief subnav linking to the four doc pages.

Author: djankov

docs/index.md

@@ -3,18 +3,21 @@
 A GitHub Action that installs [NASA GMAT](https://gmat.gsfc.nasa.gov/) and bootstraps `gmatpy` for use in CI workflows.
 
 ```yaml
-- uses: astro-tools/setup-gmat@v1
+- uses: actions/setup-python@v5
+  with:
+    python-version: '3.12'
+- uses: astro-tools/setup-gmat@v0.1
   with:
     version: R2026a
-- run: python -c "import gmatpy"
 ```
 
-A canonical Docker image on GitHub Container Registry is published alongside the action for ad-hoc local use and for downstream container layering.
+## Status
 
-```bash
-docker run --rm -it ghcr.io/astro-tools/gmat:R2026a python -c "import gmatpy"
-```
+v0.1 is the first usable release: GMAT R2026a on Linux runners (`ubuntu-latest`), caching across runs, and a built-in smoke check that loads and runs a stock GMAT sample. Windows and macOS support and the R2022a–R2026a version matrix are scoped for v0.2; see the [README roadmap](https://github.com/astro-tools/setup-gmat#roadmap) for the full plan.
 
-## Status
+## Where to next
 
-setup-gmat is under active development. The action is not yet usable.
+- [Getting started](getting-started.md) — minimum workflow, prerequisites, and how to call `gmatpy` from your own steps.
+- [Inputs and outputs](inputs-outputs.md) — full reference for `action.yml`, including cache key shape and install layout.
+- [FAQ](faq.md) — Python prerequisite, cache invalidation, supported versions.
+- [Troubleshooting](troubleshooting.md) — known failure modes mapped to the action's error messages.