doc: explain using lib.stylix.colors

The attrset is used heavily in styllix's modules, but the docs don't contain even a top-level explanation of its value. Though you can gather most it from skimming `base16.nix`, I think stylix should provide an entry point for users, interested in using their color schemes outside of stylix. I base this assumption on personal anecdote of people not having a good understanding of exported values even after extended usage and implementing parallel parsing logic for color scheme files.

docs/src/configuration.md

@@ -69,6 +69,30 @@ and the user-provided one are used in the user configuration if
 `stylix.base16Scheme` is not changed in the user config. If that is the case,
 only the user override is used.
 
+### Accessing
+
+When passing colors to unsupported targets, it is possible to access generated
+color scheme through `lib.stylix.colors`. It contains an attrset, populated by
+[base16.nix](https://github.com/SenchoPens/base16.nix)'s `mkSchemeAttrs`. TLDR
+on its contents is
+```nix
+lib.stylix.colors = {
+  base00 = "110021";
+  # ...
+  black = "110021";
+  # ...
+  withHashtag = {
+    base00 = "#110021";
+    # ... i.e. same as the parent but with a hashtag.
+  };
+};
+```
+
+For more complex configurations you may find it simpler to use
+[mustache](http://mustache.github.io/) templates to generate output files.
+See [base16.nix](https://github.com/SenchoPens/base16.nix) documentation for
+usage examples.
+
 ## Fonts
 
 The default combination of fonts is:
@@ -161,3 +185,4 @@ Targets are different between Home Manager and NixOS, and sometimes available
 in both cases. If both are available, it is always correct to enable both.
 The reference pages have a list of targets for [NixOS](options/nixos.md) and
 [Home Manager](options/hm.md) respectively.
+