hoist last-known-good to docs sidebar

docs/_templates/demo.html

@@ -30,3 +30,31 @@
     </p>
   </div>
 </div>
+
+{% if last_known_good_links %}
+<br/>
+
+<div class="work-sidebar">
+  <div class="work-sidebar-header">
+    <i class="fas fa-square-arrow-up-right"></i>
+    last-known-good preview site
+  </div>
+  <div class="work-sidebar-content">
+    <blockquote>
+      The current build seems to have failed, but a last-known-good site is available.
+    </blockquote>
+    <p>
+      {% for frag, info in lite_links.items() %}
+        <a
+          href="{{ pathto('last-known-good/' ~ frag ~ '/index') }}"
+          class="btn btn-primary"
+          target="_blank"
+          title="Try {{ info.label }} in your browser."
+        >
+          <i class="{{ info.icon }}"></i> {{ info.label }}
+        </a>
+      {% endfor %}
+    </p>
+  </div>
+</div>
+{% endif %}

docs/conf.py

@@ -22,6 +22,7 @@
 REPO_INFO = re.search(RE_GH, PROJ_DATA["project"]["urls"]["Source"])
 NOW = datetime.datetime.now(tz=datetime.timezone.utc).date()
 WORK_DIST = HERE / "_static/work"
+LAST_KNOWN_GOOD = HERE / "_static/last-known-good"
 JUPYAK_CONF_CANDIDATES = [
     ROOT / f"jupyak_config.{ext}"
     for ext in ["toml", "yaml", "yml", "json"]
@@ -103,10 +104,8 @@
     html_context["prjsf_url"] = f"{gh_pages}/_static/prjsf/prjsf.js"
 
 if ALLOW_NO_CONFIG or JUPYAK_CONF_CANDIDATES:
-    html_sidebars["*"] = [
-        "demo",
-        *html_sidebars["*"],
-    ]
+    if "demo" not in html_sidebars["*"]:
+        html_sidebars["*"] = ["demo", *html_sidebars["*"]]
     if WORK_DIST.exists():
         html_context["lite_links"] = {
             "lab": {"label": "Lab", "icon": "fas fa-flask"},
@@ -118,3 +117,18 @@
             f"preview/{frag}/index": f"_static/work/lite/{frag}/index"
             for frag in html_context["lite_links"]
         }
+
+if LAST_KNOWN_GOOD.exists():
+    if "demo" not in html_sidebars["*"]:
+        html_sidebars["*"] = ["demo", *html_sidebars["*"]]
+
+    html_context["last_known_good_links"] = {
+        "lab": {"label": "Lab", "icon": "fas fa-flask"},
+        "repl": {"label": "Console", "icon": "fas fa-terminal"},
+        "tree": {"label": "File Tree", "icon": "fas fa-folder-tree"},
+    }
+
+    rediraffe_redirects = {
+        f"last-known-good/{frag}/index": f"_static/last-known-good/{frag}/index"
+        for frag in html_context["last_known_good_links"]
+    }