fix(docs): Remove unused dotenv dependency from docusaurus config (#12102)

* fix(docs): Remove unused dotenv dependency from docusaurus config

The dotenv package was being required in docusaurus.config.js but was listed as
a devDependency, causing build failures. Since no environment variables are
actually used in the config, removed the unnecessary import.

* fix(docs): Remove reference to non-existent spending_monitoring doc

The sidebars.js file was referencing proxy/spending_monitoring which was deleted
in commit ba7463b9c. This was causing the documentation build to fail with missing
document errors.

docs/my-website/docusaurus.config.js

@@ -1,6 +1,5 @@
 // @ts-check
 // Note: type annotations allow type checking and IDEs autocompletion
-require('dotenv').config();
 
 // @ts-ignore
 const lightCodeTheme = require('prism-react-renderer/themes/github');

docs/my-website/sidebars.js

@@ -106,7 +106,6 @@ const sidebars = {
             "proxy/model_management",
             "proxy/health",
             "proxy/debugging",
-            "proxy/spending_monitoring",
             "proxy/master_key_rotations",
           ],
         },