docs/generic-device-packages: new page

Start the documentation for generic device packages and document how to
handle device-specific services. This is a question that came up in the
review of PMCR-0009 (new main device category).

Signed-off-by: Oliver Smith <ollieparanoid@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8122>

docs/generic-device-packages.md

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+# Generic Device Packages
+
+This article is about device packages starting with `device-generic-`. These
+can boot on multiple devices of that architecture. `device-generic-x86_64`
+could for example be booted on most PCs and laptops with that architecture.
+
+## Device-specific services
+
+Pre-installing device specific software is allowed if it is needed to enable
+device functionality, but it must only start on relevant devices and not
+break other devices.
+
+One example is the
+[hexagonrpcd](https://github.com/linux-msm/hexagonrpc) daemon for serving
+HexagonFS files to the DSPs on Qualcomm devices, which we only run if e.g.
+`/dev/fastrpc-adsp` is available.