Hugo fiddling and proxy docs

* Add .hugo_build.lcok to .gitignore.
* Update hugo.yaml to allow HTML elements within Markdown source files.
* Add a custom shortcode for wrapping placeholders in code snippets in <var> tags.
* Add two shortcodes to store the URL paths to the new and legacy proxy documentation.
* Add custom CSS for putting a non-copyable '$ ' prompt before console-command illustrations.
* Add documentation pages for proxy usage

.gitignore

@@ -2,3 +2,4 @@
 /resources
 .DS_Store
 *.swp
+.hugo_build.lock

hugo.yaml

@@ -6,3 +6,9 @@ menu:
   main:
     - name: Home
       url: "https://www.memcached.org/"
+
+# Render HTML elements within Markdown source files
+markup:
+  goldmark:
+    renderer:
+      unsafe: true

layouts/shortcodes/legacy_proxy_base_path.html

@@ -0,0 +1 @@
+/features/proxy/

layouts/shortcodes/proxy_base_path.html

@@ -0,0 +1 @@
+/features/proxy2/

layouts/shortcodes/var.html

@@ -0,0 +1 @@
+<var>{{.Inner}}</var>

themes/hextra/assets/css/custom.css

@@ -0,0 +1,13 @@
+/* Custom CSS for Memcached documentation */
+
+/*******/
+/* Command-line commands: Insert a non-copyable "$ " before each commmand. */
+/* This needs two selectors to do right. */
+/*******/
+code.language-console>span:not(:empty):has(span:empty):before {
+  content: "$ ";
+}
+
+code.language-console>span:not(:has(>span>span)):before {
+  content: "$ ";
+}