document supported targets

This is also a good spot to link to the pre-built binaries. And since I
did that, I can also remove the bit about not publishing binary builds
from the introduction section.

book/installing/supported-targets.md

@@ -0,0 +1,37 @@
+# Supported targets
+
+<!-- Keep this table's rows sorted -->
+<!-- markdownlint-disable-next-line MD013 MD033 -->
+| Architecture | Vendor | OS | libc | Linkage | Tier | Availability[^1]<sup>, </sup>[^2] |
+|-|-|-|-|-|-|-|
+| aarch64 | unknown | linux | musl | static | 2 | Nix, [Download](https://gitlab.computer.surgery/api/v4/projects/matrix%2Fgrapevine/jobs/artifacts/main/raw/grapevine-static-aarch64-unknown-linux-musl?job=artifacts) |
+| x86_64 | unknown | linux | glibc | dynamic | 1 | Nix |
+| x86_64 | unknown | linux | musl | static | 2 | Nix, [Download](https://gitlab.computer.surgery/api/v4/projects/matrix%2Fgrapevine/jobs/artifacts/main/raw/grapevine-static-x86_64-unknown-linux-musl?job=artifacts) |
+
+[^1]: All download links refer to the latest build of the `main` branch.
+[^2]: All targets can theoretically also be built from source without Nix.
+      However, this may require spending several hours debugging build systems.
+
+## Target tiers
+
+The "Tier" column for each target indicates the level of support that target
+has. Below is an explanation of what each tier means.
+
+### Tier 1
+
+Tier 1 targets are guaranteed to:
+
+* Build
+* Pass the test suite
+
+### Tier 2
+
+Tier 2 targets are guaranteed to:
+
+* Build
+
+## Unsupported targets
+
+Targets that don't appear in the table at the top of this page are unsupported.
+At any given time, such targets may or may not build, and may or may not pass
+the test suite.