docs: take defaults from environment variables

When building with --impure, accept the following environment variables
for docs inputs:
- NIXVIM_DOCS_BASE_HREF → baseHref
- NIXVIM_DOCS_AVAILABLE_VERSIONS → availableVersions

This allows simplifying the `website` CI workflow.

docs/mdbook/default.nix

@@ -1,3 +1,4 @@
+# NOTE: to use defaults from the environment, you must build with --impure
 {
   pkgs,
   callPackage,
@@ -9,10 +10,10 @@
   search,
   lib-docs,
   # The root directory of the site
-  baseHref ? "/",
+  baseHref ? lib.maybeEnv "NIXVIM_DOCS_BASE_HREF" "/",
   # A list of all available docs that should be linked to
   # Each element should contain { branch; nixpkgsBranch; baseHref; status; }
-  availableVersions ? [ ],
+  availableVersions ? builtins.fromJSON (lib.maybeEnv "NIXVIM_DOCS_AVAILABLE_VERSIONS" "[]"),
 }:
 let
   inherit (configuration.config.meta) nixvimInfo;