docs: Add calver to semver banner

All Merlin projects are switching from semantic versioning,
like `0.4.0`, to calendar versioning, like `23.01`.
The change is planned for January 2023.
Add a banner to all doc pages to prepare our consumers
for the change.

docs/source/_static/css/custom.css

@@ -0,0 +1,34 @@
+.wy-nav-content {
+    margin: 0;
+    background: #fcfcfc;
+    padding-top: 40px;
+}
+
+.wy-side-nav-search {
+    display: block;
+    width: 300px;
+    padding: .809em;
+    padding-top: 0.809em;
+    margin-bottom: .809em;
+    z-index: 200;
+    background-color: #2980b9;
+    text-align: center;
+    color: #fcfcfc;
+    padding-top: 40px;
+}
+
+div.banner {
+    position: fixed;
+    top: 10px;
+    left: 20px;
+    margin: 0;
+    z-index: 1000;
+    width: 1050px;
+    text-align: center;
+}
+
+p.banner {
+  border-radius: 4px;
+  color: #004831;
+  background: #76b900;
+}

docs/source/_templates/layout.html

@@ -0,0 +1,9 @@
+{% extends "!layout.html" %}
+{% block extrabody %}
+  <div class="banner">
+    <p class="banner">
+      Beginning in January 2023, versions for all NVIDIA Merlin projects
+      will change from semantic versioning like <code>4.0</code>
+      to calendar versioning like <code>23.01</code>.</p>
+  </div>
+{% endblock %}

docs/source/conf.py

@@ -103,7 +103,8 @@
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,
 # so a file named "default.css" will overwrite the builtin "default.css".
-html_static_path = []
+html_static_path = ["_static"]
+html_css_files = ["css/custom.css"]
 
 source_suffix = [".rst", ".md"]