Use alternate way to set collection section pages' layout to single

Under the `collections` section, this website has been using the
directory structure in a different way from Hugo's assumption on
sections.  Hugo treats a sub-section's _index.md differently from other
pages: _index.md uses a section template rather than a single template.
However, this website designates a sub-section's _index.md as an equal
page to other pages with regards to the template applied.

The old way this website used to render an _index.md under the
`collections` section using a single template is to call
`.Render "single"` in the section template applied to the `collections`
section (`layouts/_default/collections.html`).  However, this way has
stopped working since Hugo v0.146.2.

Therefore, it is now necessary to switch to a new way to achieve this
effect: under the `collections` section, for each page with the
`section` kind, set `layout: single` in the page's front matter.
Nevertheless, this alternate way is probably clearer and less quirky
even if the old way was not broken.

Author: Leo3418

config/_default/hugo.yaml

@@ -25,6 +25,8 @@ cascade:
   - _target:
       path: '/collections{,/**}'
       kind: 'section'
+    # Render a collection sub-section's landing page as a regular page
+    layout: 'single'
     outputs:
       # Drop 'RSS'
       - 'HTML'