docs: add version warning message

and fix font sizes+weights of the admonition component

docs/_static/styles/custom.css

@@ -12,6 +12,8 @@
   --color-brand-content: rgb(0, 115, 189);  /* bright icon color */
   /* misc */
   --sidebar-item-spacing-vertical: .4rem;
+  --admonition-font-size: var(--font-size--small);
+  --admonition-title-font-size: var(--font-size--normal);
 }
 
 @media (prefers-color-scheme: dark) {
@@ -115,6 +117,17 @@ strong.command {
   Components
 */
 
+.admonition p.admonition-title {
+  font-weight: bold;
+}
+.admonition.version-warning {
+  padding-bottom: 0;
+}
+.admonition.version-warning > .admonition-title {
+  margin-bottom: 0;
+  font-weight: normal;
+}
+
 table.table-custom-layout {
   width: 100%;
   table-layout: fixed;

docs/_templates/page.html

@@ -0,0 +1,9 @@
+{% extends '!page.html' %}
+{% block content %}
+  {% if release != version %}
+    <div class="admonition important version-warning">
+      <p class="admonition-title">You are reading the documentation for the in-development version of Streamlink.</p>
+    </div>
+  {% endif %}
+  {{ super() }}
+{% endblock %}