build(docs): support YAML frontmatter in markdown

Vitepress uses YAML frontmatter to configure Vitepress specific
settings, see https://vitepress.vuejs.org/config/frontmatter-configs

We just need to use `remark-frontmatter` to add support for it.

GitHub also renders the YAML front-matter nicely in a table
automatically, but maybe we should instead strip it, if it's only
used by Vitepress?

pnpm-lock.yaml

@@ -275,6 +275,9 @@ importers:
       remark:
         specifier: ^14.0.2
         version: 14.0.2
+      remark-frontmatter:
+        specifier: ^4.0.1
+        version: 4.0.1
       remark-gfm:
         specifier: ^3.0.1
         version: 3.0.1
@@ -3970,7 +3973,7 @@ packages:
   /axios/0.21.4_debug@4.3.2:
     resolution: {integrity: sha512-ut5vewkiu8jjGBdqpM44XxjuCjq9LAKeHVmoVfHVzy8eHgxxq8SbAVQNovDA8mVi05kP0Ea/n/UzcSHcTJQfNg==}
     dependencies:
-      follow-redirects: 1.15.2_debug@4.3.2
+      follow-redirects: 1.15.2
     transitivePeerDependencies:
       - debug
     dev: true
@@ -6663,6 +6666,12 @@ packages:
       reusify: 1.0.4
     dev: true
 
+  /fault/2.0.1:
+    resolution: {integrity: sha512-WtySTkS4OKev5JtpHXnib4Gxiurzh5NCGvWrFaZ34m6JehfTUhKZvn9njTfw48t6JumVQOmrKqpmGcdwxnhqBQ==}
+    dependencies:
+      format: 0.2.2
+    dev: true
+
   /faye-websocket/0.11.4:
     resolution: {integrity: sha512-CzbClwlXAuiRQAlUyfqPgvPoNKTckTPGfwZV4ZdAhVcP2lh9KUxJg2b5GkE7XbjKQ3YJnQ9z6D9ntLAlB+tP8g==}
     engines: {node: '>=0.8.0'}
@@ -6762,7 +6771,7 @@ packages:
     resolution: {integrity: sha512-XGozTsMPYkm+6b5QL3Z9wQcJjNYxp0CYn3U1gO7dwD6PAqU1SVWZxI9CCg3z+ml3YfqdPnrBehaBrnH2AGKbNA==}
     dev: true
 
-  /follow-redirects/1.15.2_debug@4.3.2:
+  /follow-redirects/1.15.2:
     resolution: {integrity: sha512-VQLG33o04KaQ8uYi2tVNbdrWp1QWxNNea+nmIB4EVM28v0hmP17z7aG1+wAkNzVq4KeXTq3221ye5qTJP91JwA==}
     engines: {node: '>=4.0'}
     peerDependencies:
@@ -6770,8 +6779,6 @@ packages:
     peerDependenciesMeta:
       debug:
         optional: true
-    dependencies:
-      debug: 4.3.2
     dev: true
 
   /foreground-child/2.0.0:
@@ -6813,6 +6820,11 @@ packages:
       mime-types: 2.1.35
     dev: true
 
+  /format/0.2.2:
+    resolution: {integrity: sha512-wzsgA6WOq+09wrU1tsJ09udeR/YZRaeArL9e1wPbFg3GG2yDnC2ldKpxs4xunpFF9DgqCqOIra3bc1HWrJ37Ww==}
+    engines: {node: '>=0.4.x'}
+    dev: true
+
   /forwarded/0.2.0:
     resolution: {integrity: sha512-buRG0fpBtRHSTCOASe6hD258tEubFoRLb4ZNA6NxMVHNw2gOcwHo9wyablzMzOA5z9xA9L1KNjk/Nt6MT9aYow==}
     engines: {node: '>= 0.6'}
@@ -7306,7 +7318,7 @@ packages:
     engines: {node: '>=8.0.0'}
     dependencies:
       eventemitter3: 4.0.7
-      follow-redirects: 1.15.2_debug@4.3.2
+      follow-redirects: 1.15.2
       requires-port: 1.0.0
     transitivePeerDependencies:
       - debug
@@ -8747,6 +8759,12 @@ packages:
       - supports-color
     dev: true
 
+  /mdast-util-frontmatter/1.0.0:
+    resolution: {integrity: sha512-7itKvp0arEVNpCktOET/eLFAYaZ+0cNjVtFtIPxgQ5tV+3i+D4SDDTjTzPWl44LT59PC+xdx+glNTawBdF98Mw==}
+    dependencies:
+      micromark-extension-frontmatter: 1.0.0
+    dev: true
+
   /mdast-util-gfm-autolink-literal/1.0.2:
     resolution: {integrity: sha512-FzopkOd4xTTBeGXhXSBU0OCDDh5lUj2rd+HQqG92Ld+jL4lpUfgX2AT2OHAVP9aEeDKp7G92fuooSZcYJA3cRg==}
     dependencies:
@@ -8917,6 +8935,14 @@ packages:
       uvu: 0.5.6
     dev: true
 
+  /micromark-extension-frontmatter/1.0.0:
+    resolution: {integrity: sha512-EXjmRnupoX6yYuUJSQhrQ9ggK0iQtQlpi6xeJzVD5xscyAI+giqco5fdymayZhJMbIFecjnE2yz85S9NzIgQpg==}
+    dependencies:
+      fault: 2.0.1
+      micromark-util-character: 1.1.0
+      micromark-util-symbol: 1.0.1
+    dev: true
+
   /micromark-extension-gfm-autolink-literal/1.0.3:
     resolution: {integrity: sha512-i3dmvU0htawfWED8aHMMAzAVp/F0Z+0bPh3YrbTPPL1v4YAlCZpy5rBO5p0LPYiZo0zFVkoYh7vDU7yQSiCMjg==}
     dependencies:
@@ -10148,6 +10174,15 @@ packages:
       jsesc: 0.5.0
     dev: true
 
+  /remark-frontmatter/4.0.1:
+    resolution: {integrity: sha512-38fJrB0KnmD3E33a5jZC/5+gGAC2WKNiPw1/fdXJvijBlhA7RCsvJklrYJakS0HedninvaCYW8lQGf9C918GfA==}
+    dependencies:
+      '@types/mdast': 3.0.10
+      mdast-util-frontmatter: 1.0.0
+      micromark-extension-frontmatter: 1.0.0
+      unified: 10.1.2
+    dev: true
+
   /remark-gfm/3.0.1:
     resolution: {integrity: sha512-lEFDoi2PICJyNrACFOfDD3JlLkuSbOa5Wd8EPt06HUdptv8Gn0bxYTdbU/XXQ3swAPkEaGxxPN9cbnMHvVu1Ig==}
     dependencies: