chore: use basePath from metadata.json for mise run generate-docs

[`vars` can also be read in as a file. (mise.jdx.dev)](https://mise.jdx.dev/tasks/task-configuration.html#vars-options).

Continues typst-jp#303.
Relates to typst-jp#233.

Author: YDX-2147483647

.mise.toml

@@ -8,8 +8,11 @@ pinact = "3.1.1"
 yes = true
 idiomatic_version_file_enable_tools = []
 
+[vars]
+_.file = "website/metadata.json"
+
 [tasks.generate-docs]
-run = "cargo run --package typst-docs -- --assets-dir assets --out-file docs.json --base /docs/"
+run = "cargo run --package typst-docs -- --assets-dir assets --out-file docs.json --base {{vars.basePath}}"
 
 [tasks.generate-web]
 depends = ["install-website"]