add under-construction admonition; document custom admonitions in contributing

Author: bcumming

docs/alps/hardware.md

@@ -51,6 +51,11 @@ There are currently four node types in Alps, with another becoming available in
 [](){#ref-alps-gh200-node}
 ### NVIDIA GH200 GPU Nodes
 
+!!! under-construction
+    The description of the GH200 nodes is incomplete.
+
+    Let us know if there is missing information.
+
 There are 24 cabinets, in 4 rows with 6 cabinets per row:
 
 * 8 chassis per cabinet

docs/contributing/index.md

@@ -229,6 +229,66 @@ They stand out better from the main text, and can be collapsed by default if nee
 
 If an admonition is collapsed by default, it should have a title.
 
+We provide some custom admonitions.
+
+#### Change
+
+For adding information about a change, originally designed for recording updates to clusters.
+
+=== "Rendered"
+    !!! change "2025-04-17"
+        * SLURM was upgraded to version 25.1.
+        * uenv was upgraded to v0.8
+
+    Old changes can be folded:
+
+    ??? change "2025-02-04"
+        * The new Scratch cleanup policy was implemented
+        * NVIDIA driver was updated
+
+=== "Markdown"
+    ```
+    !!! change "2025-04-17"
+        * SLURM was upgraded to version 25.1.
+        * uenv was upgraded to v0.8
+    ```
+
+    Old changes can be folded:
+
+    ```
+    ??? change "2025-02-04"
+        * The new Scratch cleanup policy was implemented
+        * NVIDIA driver was updated
+    ```
+
+#### Under construction
+
+For marking incomplete sections.
+
+=== "Rendered"
+    !!! under-construction
+        This is not finished yet!
+
+=== "Markdown"
+    ```
+    !!! under-construction
+        This is not finished yet!
+    ```
+
+#### Todo
+
+As a placeholder for documentation that needs to be written.
+
+=== "Rendered"
+    !!! todo
+        Add some common error messages and how to fix them.
+
+=== "Markdown"
+    ```
+    !!! todo
+        Add some common error messages and how to fix them.
+    ```
+
 ### Code blocks
 
 Use [code blocks](https://squidfunk.github.io/mkdocs-material/reference/code-blocks/) when you want to display monospace text in a programming language, terminal output, configuration files etc.

docs/stylesheets/extra.css

@@ -53,6 +53,22 @@
           mask-image: var(--md-admonition-icon--alps);
 }
 
+/* under-construction admonition */
+.md-typeset .admonition.under-construction,
+.md-typeset details.under-construction {
+  border-color: rgb(0, 0, 0);
+}
+.md-typeset .under-construction > .admonition-title,
+.md-typeset .under-construction > summary {
+  background-color: rgba(255, 255, 0, 0.3);
+}
+.md-typeset .under-construction > .admonition-title::before,
+.md-typeset .under-construction > summary::before {
+  background-color: rgb(0, 0, 0);
+  -webkit-mask-image: var(--md-admonition-icon--digging);
+          mask-image: var(--md-admonition-icon--digging);
+}
+
 .md-nav__item .md-nav__link--active {
     font-weight: bold;
 }