Add workaround for Hugo PublishDate regression

Add a custom `rss.xml` default layout with a workaround for a
`PublishDate` regression for Hugo v0.125 and newer where the
`PublishDate` does not get set correctly for all generated files.
Details can be found here:

gohugoio/hugo#12438

Hugo documentation:

https://gohugo.io/templates/rss/#template-lookup-order

The `rss.xml` file is based on the upstream default template found here:

```
curl -LO https://raw.githubusercontent.com/gohugoio/hugo/3d40aba512931031921463dafc172c0d124437b8/tpl/tplimpl/embedded/templates/_default/rss.xml
```

The changed line is:

```
<pubDate>{{ (or .PublishDate .Date).Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
```

layouts/_default/rss.xml

@@ -0,0 +1,71 @@
+{{- /* Deprecate site.Author.email in favor of site.Params.author.email */}}
+{{- $authorEmail := "" }}
+{{- with site.Params.author }}
+  {{- if reflect.IsMap . }}
+    {{- with .email }}
+      {{- $authorEmail = . }}
+    {{- end }}
+  {{- end }}
+{{- else }}
+  {{- with site.Author.email }}
+    {{- $authorEmail = . }}
+    {{- warnf "The author key in site configuration is deprecated. Use params.author.email instead." }}
+  {{- end }}
+{{- end }}
+
+{{- /* Deprecate site.Author.name in favor of site.Params.author.name */}}
+{{- $authorName := "" }}
+{{- with site.Params.author }}
+  {{- if reflect.IsMap . }}
+    {{- with .name }}
+      {{- $authorName = . }}
+    {{- end }}
+  {{- else }}
+    {{- $authorName  = . }}
+  {{- end }}
+{{- else }}
+  {{- with site.Author.name }}
+    {{- $authorName = . }}
+    {{- warnf "The author key in site configuration is deprecated. Use params.author.name instead." }}
+  {{- end }}
+{{- end }}
+
+{{- $pctx := . }}
+{{- if .IsHome }}{{ $pctx = .Site }}{{ end }}
+{{- $pages := slice }}
+{{- if or $.IsHome $.IsSection }}
+{{- $pages = $pctx.RegularPages }}
+{{- else }}
+{{- $pages = $pctx.Pages }}
+{{- end }}
+{{- $limit := .Site.Config.Services.RSS.Limit }}
+{{- if ge $limit 1 }}
+{{- $pages = $pages | first $limit }}
+{{- end }}
+{{- printf "<?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"yes\"?>" | safeHTML }}
+<rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
+  <channel>
+    <title>{{ if eq .Title .Site.Title }}{{ .Site.Title }}{{ else }}{{ with .Title }}{{ . }} on {{ end }}{{ .Site.Title }}{{ end }}</title>
+    <link>{{ .Permalink }}</link>
+    <description>Recent content {{ if ne .Title .Site.Title }}{{ with .Title }}in {{ . }} {{ end }}{{ end }}on {{ .Site.Title }}</description>
+    <generator>Hugo</generator>
+    <language>{{ site.Language.LanguageCode }}</language>{{ with $authorEmail }}
+    <managingEditor>{{.}}{{ with $authorName }} ({{ . }}){{ end }}</managingEditor>{{ end }}{{ with $authorEmail }}
+    <webMaster>{{ . }}{{ with $authorName }} ({{ . }}){{ end }}</webMaster>{{ end }}{{ with .Site.Copyright }}
+    <copyright>{{ . }}</copyright>{{ end }}{{ if not .Date.IsZero }}
+    <lastBuildDate>{{ (index $pages.ByLastmod.Reverse 0).Lastmod.Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</lastBuildDate>{{ end }}
+    {{- with .OutputFormats.Get "RSS" }}
+    {{ printf "<atom:link href=%q rel=\"self\" type=%q />" .Permalink .MediaType | safeHTML }}
+    {{- end }}
+    {{- range $pages }}
+    <item>
+      <title>{{ .Title }}</title>
+      <link>{{ .Permalink }}</link>
+      <pubDate>{{ (or .PublishDate .Date).Format "Mon, 02 Jan 2006 15:04:05 -0700" | safeHTML }}</pubDate>
+      {{- with $authorEmail }}<author>{{ . }}{{ with $authorName }} ({{ . }}){{ end }}</author>{{ end }}
+      <guid>{{ .Permalink }}</guid>
+      <description>{{ .Summary | transform.XMLEscape | safeHTML }}</description>
+    </item>
+    {{- end }}
+  </channel>
+</rss>