Docs: serve markdown files via Firebase instead of jsdelivr

Might make a few corporate web proxies happier

docs/firebase.json

@@ -6,7 +6,6 @@
       ".*",
       "build.js",
       "firebase.json",
-      "*.md",
       "image/**",
       "search-index/**"
     ],

docs/index.html

@@ -7,7 +7,7 @@
   <meta name="description" content="Resize large images in common formats to smaller, web-friendly JPEG, PNG, WebP and AVIF images of varying dimensions">
   <meta http-equiv="Content-Security-Policy" content="default-src 'self'; object-src 'none'; style-src 'unsafe-inline';
     img-src 'unsafe-inline' data: https://pixel.plumbing/px/ https://cdn.jsdelivr.net/gh/lovell/ https://www.google-analytics.com;
-    connect-src 'self' https://cdn.jsdelivr.net/gh/lovell/ https://www.google-analytics.com;
+    connect-src 'self' https://www.google-analytics.com;
     script-src 'unsafe-inline' 'unsafe-eval'
       https://cdn.jsdelivr.net/npm/docute@4/dist/docute.min.js
       https://www.google-analytics.com/analytics.js;">
@@ -19,8 +19,6 @@
   <link rel="apple-touch-icon-precomposed" sizes="72x72" href="https://pixel.plumbing/px/72x72/sharp-logo.svg">
   <link rel="apple-touch-icon-precomposed" href="https://pixel.plumbing/px/57x57/sharp-logo.svg">
   <link rel="author" href="/humans.txt" type="text/plain">
-  <link rel="preload" href="https://cdn.jsdelivr.net/gh/lovell/sharp@v0.28.3/docs/README.md" as="fetch" type="text/markdown" crossorigin>
-  <link rel="preload" href="https://cdn.jsdelivr.net/gh/lovell/sharp@master/docs/image/sharp-logo.svg" as="image" type="image/svg+xml" crossorigin>
   <link rel="dns-prefetch" href="https://pixel.plumbing">
   <link rel="dns-prefetch" href="https://www.google-analytics.com">
   <script type="application/ld+json">
@@ -139,7 +137,6 @@
         docuteApiTitlePlugin,
         docuteApiSearchPlugin
       ],
-      sourcePath: 'https://cdn.jsdelivr.net/gh/lovell/sharp@v0.28.3/docs',
       nav: [
         {
           title: 'Funding',