docs: add platform support page in the docs

docs/src/platforms.md

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+# Platform support
+
+<!-- markdownlint-disable MD033 -->
+
+uutils aims to be as "universal" as possible, meaning that we try to support
+many platforms. However, it is infeasible for us to guarantee that every
+platform works. Just like Rust itself, we therefore have multiple tiers of
+platform support, with different guarantees. We support two tiers of platforms:
+
+ - **Tier 1**: All applicable utils are compiled and tested in CI for these
+   platforms.
+ - **Tier 2**: These platforms are supported but not actively tested. We do accept
+   fixes for these platforms.
+
+> **Note**: The tiers are dictated by our CI. We would happily accept a job
+> in the CI for testing more platforms, bumping those platforms to tier 1.
+
+## Platforms per tier
+
+The platforms in tier 1 and the platforms that we test in CI are listed below.
+
+| Operating system | Tested targets |
+| ---------------- | -------------- |
+| **Linux**        | `x86_64-unknown-linux-gnu` <br> `x86_64-unknown-linux-musl` <br> `arm-unknown-linux-gnueabihf` <br> `i686-unknown-linux-gnu` <br> `aarch64-unknown-linux-gnu` |
+| **macOS**        | `x86_64-apple-darwin` |
+| **Windows**      | `i686-pc-windows-msvc` <br> `x86_64-pc-windows-gnu` <br> `x86_64-pc-windows-msvc` |
+| **FreeBSD**      | `x86_64-unknown-freebsd` |
+| **Android**      | `i686-linux-android` |
+
+The platforms in tier 2 are more vague, but include:
+
+ - untested variations of the platforms above,
+ - Redox OS,
+ - and BSDs such as OpenBSD, NetBSD & DragonFlyBSD.
+
+## Utility compatibility per platform
+
+Not all utils work on every platform. For instance, `chgrp` is not supported on
+Windows, because Windows does have the concept of groups. Below is a full table
+detailing which utilities are supported for the tier 1 platforms.
+
+Note that for some utilities, not all functionality is supported on each
+platform. This is documented per utility.
+
+{{ #include platform_table.md }}