Document using doas instead of sudo for Alpine Installation method

This issue has been migrated from [#16391](https://github.com/matrix-org/synapse/issues/16391).

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`sudo` *must* be explicitly installed by the user (or through a dependency), so it will not be available. Alpine Linux actually uses `doas` instead of `sudo`.

[doas](https://pkgs.alpinelinux.org/packages?name=doas&branch=edge&repo=&arch=&maintainer=) is part of the main repository, and [sudo](https://pkgs.alpinelinux.org/packages?name=sudo&branch=edge&repo=&arch=&maintainer=) is only part of the community package. (See [Repositories](https://wiki.alpinelinux.org/wiki/Repositories) for difference between `main` and `community` repo)

doas is installed when necessary (unprivileged user setup during `setup-alpine`), sudo is only installed either explicitly by the user or indirectly through a package that depends on it (2 of them), so sudo will not be available and will cause problems if the command is copied as is.

I'd suggest changing `sudo` to `doas` for the Alpine Linux install.

Also, the community repository needs to be enabled explicitly too, which isn't part of the instruction.