UX: Applying more admin UI guidelines (#956)

This commit applies further admin UI guidelines, now that they have been more
fleshed out in core, to the AI admin UI:

* Tools
* LLMs
* Personas

The changes include but are not limited to:

* Applying the table CSS classes, for desktop and mobile
* Adding a description and learn more link for each tab
* Adding an empty list placeholder with CTA using `AdminConfigAreaEmptyList`
* Replacing custom headings with `AdminPageSubheader`

admin/assets/javascripts/discourse/templates/admin-plugins/show/discourse-ai-tools/new.hbs

@@ -1,5 +1,7 @@
-<AiToolEditor
-  @tools={{this.allTools}}
-  @model={{this.model}}
-  @presets={{this.presets}}
-/>
+<section class="ai-persona-tool-editor__current admin-detail pull-left">
+  <AiToolEditor
+    @tools={{this.allTools}}
+    @model={{this.model}}
+    @presets={{this.presets}}
+  />
+</section>

admin/assets/javascripts/discourse/templates/admin-plugins/show/discourse-ai-tools/show.hbs

@@ -1,5 +1,7 @@
-<AiToolEditor
-  @tools={{this.allTools}}
-  @model={{this.model}}
-  @presets={{this.presets}}
-/>
+<section class="ai-persona-tool-editor__current admin-detail pull-left">
+  <AiToolEditor
+    @tools={{this.allTools}}
+    @model={{this.model}}
+    @presets={{this.presets}}
+  />
+</section>