docs: Outline Jsonnet language (#19410)

This PR adds a basic documentation about the Jsonnet language support.

Release Notes:

- N/A

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

docs/src/languages/jsonnet.md

@@ -0,0 +1,24 @@
+# Jsonnet
+
+Jsonnet language support in Zed is provided by the community-maintained [Jsonnet extension](https://github.com/narqo/zed-jsonnet).
+
+- Tree Sitter: [sourcegraph/tree-sitter-jsonnet](https://github.com/sourcegraph/tree-sitter-jsonnet)
+- Language Server: [grafana/jsonnet-language-server](https://github.com/grafana/jsonnet-language-server)
+
+## Configuration
+
+Workspace configuration options can be passed to the language server via the `lsp` settings of the `settings.json`.
+
+The following example enables support for resolving [tanka](https://tanka.dev) import paths in `jsonnet-language-server`:
+
+```json
+{
+  "lsp": {
+    "jsonnet-language-server": {
+      "settings": {
+        "resolve_paths_with_tanka": true
+      }
+    }
+  }
+}
+```