doc: add flake description and make project synopsis more concise

[trueNAHO]

commit f6135feb96da3ab84192312831eca1dbaf24ed00
[...]

    stylix: add flake description

commit d7033fc12eef3bd0f09b87742bf78c3cd53c1197
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    doc: make project synopsis more concise

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Upon merging this PR, the GitHub project description should also be updated:

[danth]

Both the current and new descriptions fail to mention that standalone Home Manager is also supported. We actually have more Home Manager modules than NixOS. There's also Darwin to some extent, and Nix-on-Droid in the future.

Is there a way we could make this clear without the description becoming too generalised? I'm thinking of making reference to Nix rather than NixOS, but I'm not sure on the exact wording.

(We also have more than color schemes, fonts, and wallpapers - however the other things such as opacity are only supported on a small handful of targets - so I think having that list in the description is still okay.)

[trueNAHO]

Both the current and new descriptions fail to mention that standalone Home Manager is also supported. We actually have more Home Manager modules than NixOS. There's also Darwin to some extent, and Nix-on-Droid in the future.

Indeed. I omitted this information in the abstract description, since it is mentioned in the following documentation. However, adding this information in the abstract description is indeed better.

Is there a way we could make this clear without the description becoming too generalised? I'm thinking of making reference to Nix rather than NixOS, but I'm not sure on the exact wording.

While keeping the project synopsis extremely concise, we could change it to "Nix theming framework". The "framework" term implies that we integrate with other Nix frameworks like NixOS, Home Manager, and nix-darwin. The "theming" term encompasses all theming functionalities.

(We also have more than color schemes, fonts, and wallpapers - however the other things such as opacity are only supported on a small handful of targets - so I think having that list in the description is still okay.)

Despite the maintenance overhead, listing all these functionalities in the abstract is probably worthwhile, especially since we do not add new tree-wide features very frequently.

I end up with the following abstract variants:

• Stylix is a Nix theming framework for NixOS, Home Manager, and nix-darwin that
  applies color schemes, wallpapers, and fonts to a wide range of applications.
  

• Stylix is a Nix theming framework, applying color schemes, wallpapers, and
  fonts to a wide range of applications across NixOS, Home Manager, and
  nix-darwin.
  

The first variant keeps "NixOS, Home Manager, and nix-darwin" close to "framework" while giving ", applying" up for "that applies". The second variant keeps ", applying", but puts "NixOS, Home Manager, and nix-darwin" too far from "framework". The first variant is the best I can come up with for now.

I still have the following open questions:

• Should we replace "nix-darwin" with "Darwin".
• Should we add hyperlinks to each framework: "NixOS, Home Manager, and nix-darwin"?
• Should we add "opacity" after "fonts" as "opacity" or "transparency"?
• Implicit features like "border radius" are probably still far too immature to include, right?

Once we agree on the project synopsis and abstract description, we could expand the scope of this PR to overhaul the "What's this?" section.