Use Site.Params.mainSections to select pages on home page

This helps with theme portability.

Also use Site.RegularPages in its default sort, as this is probably what most people want.

layouts/index.html

   <main class="cf ph3 ph5-l pv3 pv4-l f4 tc-l center measure-wide lh-copy mid-gray">
     {{ .Content }}
   </main>
-  {{/* Create a variable to use the same section twice. Note that we've included the date here. */}}
-  {{ $section := where .Data.Pages.ByDate.Reverse "Section" "post" }}
+  {{/* For Hugo 0.20 this will default to the section with the most number of pages. */}}
+  {{ $mainSections := .Site.Params.mainSections | default (slice "post") }}
+  {{/* Create a variable to use the same section twice. */}}
+  {{ $section := where .Site.RegularPages "Section" "in" $mainSections }}
   {{ if $section }}
     <div class="pa3 pa4-ns w-100 w-70-ns center">