doc: minimal documentation of supported platforms

This commit adds minimal documentation of the supported platforms.
More exhaustive documentation would require producing a list of
platforms for each of the 7 tiers. This was attempted in #245368, but it
quickly became clear that that would be a long-term effort.

In the meantime, this commit adds the most important information to the
manual.

Co-authored-by: Valentin Gagarin <[e-mail]>

doc/using-nixpkgs.md

···
 # Using Nixpkgs {#part-using}
 
 ```{=include=} chapters
+using/platform-support.chapter.md
 using/configuration.chapter.md
 using/overlays.chapter.md
 using/overrides.chapter.md

doc/using/platform-support.chapter.md

···
+# Platform Support {#chap-platform-support}
+
+Packages receive varying degrees of support, both in terms of maintainer attention and available computation resources for continuous integration (CI).
+
+Below is the list of the best supported platforms:
+
+- `x86_64-linux`: Highest level of support.
+- `aarch64-linux`: Well supported, with most packages building successfully in CI.
+- `aarch64-darwin`: Receives better support than `x86_64-darwin`.
+- `x86_64-darwin`: Receives some support.
+
+There are many other platforms with varying levels of support.
+The provisional platform list in [Appendix A] of [RFC046], while not up to date, can be used as guidance.
+
+A more formal definition of the platform support tiers is provided in [RFC046], but has not been fully implemented yet.
+
+[RFC046]: https://github.com/NixOS/rfcs/blob/master/rfcs/0046-platform-support-tiers.md
+[Appendix A]: https://github.com/NixOS/rfcs/blob/master/rfcs/0046-platform-support-tiers.md#appendix-a-non-normative-description-of-platforms-in-november-2019