(docs) add Linux docs

docs/linux.mdx

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+## Linux support
+
+Currently, only the Dart package (`thermion_dart`) will work on Linux (primarily for the automated build/testing pipeline). 
+
+Filament has been built for Ubuntu 22.04; other distributions may work, but YMMV.
+
+The Flutter package (`thermion_flutter`) won't work; it's missing the platform glue code necessary to set up the render target.  Support will eventually be added, but there is currently no estimated timeline.
+
+Contributions are welcome and it's actually not a lot of work. Join the Discord if you want to help out.
+