Add docs for Snap

Add a page for Snaps and migrating away from it.

docs/user/software/third-party/snap.md

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+---
+title: Snap
+summary: Working with snaps
+---
+
+# Snap
+:::warning
+Note that snaps are going to be [removed from Solus in the future][2],
+and are not fully confined in Solus with Linux 6.9 and newer.
+See the section on confinement below.
+:::
+
+[Snaps][1] are a way of installing third-party packages.
+The `snapd` package installs the CLI client that can be used to install and run snaps.
+
+## Confinement warning
+
+Snaps are usually protected using `strict` confinement,
+ensuring that snaps can't access more of your system than needed.
+The `snap` command provided by Solus shows a warning when strict confinement is not available.
+For example:
+
+> WARNING: snap is running with partial confinement. See https://help.getsol.us/docs/user/software/third-party/snap for details
+
+We recommend [migrating to Flatpak](#migrating-to-flatpak) when possible.
+The warning can be silenced by running `sudo snap hide-confinement-warning`.
+
+## Migrating to Flatpak
+
+The `unsnap` package can be used to migrate from snap to Flatpak.
+While it does not actively remove user data in `~/snap` or `/var/lib/snapd`,
+we cannot guarantee that no data gets removed on accident.
+Please ensure you have system backups that include snap data.
+
+Run `sudo unsnap` before migrating to see if all your snaps have known equivalents.
+Look for lines containing `No equivalent flatpak for <name> found`.
+Check out [this issue][3] if you encounter such snaps.
+
+Running `sudo unsnap auto` will migrate as many snaps to Flatpak as possible.
+It will also uninstall `snapd` if all snaps were migrated.
+
+If `unsnap` is unable to find an equivalent Flatpak for a snap, you have the following options:
+
+- Search for an equivalent Flatpak manually, and [report it][3] if possible.
+- Install an equivalent tool from the package manager if possible.
+- Uninstall the snap manually if it is not needed.
+
+It is possible to rerun `sudo unsnap auto` to finish removing `snapd`.
+
+[1]: https://snapcraft.io/
+[2]: https://github.com/getsolus/packages/issues/325
+[3]: https://github.com/getsolus/packages/issues/3282