Update usage instructions for Flatpak

[guihkx]

This simplifies things regarding the Flatpak set up. This also adds:

• Flatpak-specific build instructions.
• Remove inaccurate comment about the Flatpak package.
• Remove every $ at the beginning of each command for easier copying and pasting.

Rendered page: https://github.com/guihkx/spotify-adblock/blob/readme-flatpak/README.md

Closes #112

[guihkx]

Apologies for the extra force-pushes. It should be ready for review now.

[axelsimon]

@guihkx small mistake in your instructions.

mkdir -p ~/.var/app/com.spotify.Client/config/spotify-adblock
but then:
cp config.toml ~/.var/app/com.spotify.Client/spotify-adblock

/config is missing, the right command is:
cp config.toml ~/.var/app/com.spotify.Client/config/spotify-adblock

Also, i'm not sure i like telling people they just need to use podman to build. At least we should give people the option: either you need cargo + rust, or if you don't want to install that you can run a container using podman (why Ubuntu 23.10 though, at least use a LTS or a recent version). But then again you can also use nix and nix-shell -p cargo so, i'm not sure we should push flatpak users to a specific solution.

[axelsimon]

Needed change.

[guihkx]

Also, i'm not sure i like telling people they just need to use podman to build.

That's necessary because building this library on a distro that has a different glibc version than the one provided by the Freedesktop Runtime (which Spotify for Flatpak runs on), may cause the library not to work (see #24 and #28).

And that's the reason why Ubuntu 23.10 was chosen: It has glibc 2.38, which is the same version present in the 23.08 Freedesktop Runtime, which was used by Spotify when I created this pull request, but now they've updated to 24.08, so I'll need to find another distro with the same glibc version...

i'm not sure we should push flatpak users to a specific solution.

I'm done because it's the solution that I tested and it's guaranteed work. People are, of course, free to choose whatever build methods they like. Or they can avoid the build process completely and try their luck with the pre-built library, but again, I can't guarantee that will work...

[guihkx]

Updates:

• Fixed the cp config.toml ... command not targeting the right location.
• Updated the Flatpak-specific build instructions to use a Ubuntu 24.10 container image, since it has glibc 2.40, which matches the version used by the runtime Spotify for Flatpak currently runs on (24.08).

[guihkx]

I'm so sorry, I deleted the branch by mistake while I was pushing changes to this other branch I'm working on.