Add new page "API Reference" to access Deno doc.

src/SUMMARY.md

@@ -17,4 +17,5 @@
   - [Properly create a virtual buffer](./tutorial/maze/properly-create-a-virtual-buffer.md)
   - [Properly configured the buffer](./tutorial/maze/properly-configured-the-buffer.md)
   - [Reduce the number of RPC calls](./tutorial/maze/reduce-the-number-of-rpc-calls.md)
+- [API Reference](./api-reference.md)
 - [FAQ](./faq.md)

src/api-reference.md

@@ -0,0 +1,10 @@
+# API Reference
+
+There is a standard module [denops_std] to develop denops plugins.
+It provides various functions to interact with Vim and Neovim and some
+shorthands to make it easier to write plugins.
+
+You can find API references about the module by checking the Deno doc
+page: `https://deno.land/x/denops_std/mod.ts`.
+
+[denops_std]: https://deno.land/x/denops_std/mod.ts