Add custom commit path for last edited link (#319)

- A new `BookCommitPath` paramter was added that allows the 'commit'
   portion of the link to a page's last edited commit hash to be
   specified.
 - This is useful when github is not the default repository for a
   project.

Co-authored-by: Collin Brooks <cbrooks@sentryds.com>

layouts/partials/docs/footer.html

@@ -6,7 +6,8 @@
 {{ if and .GitInfo .Site.Params.BookRepo }}
   <div>
     {{- $date := partial "docs/date" (dict "Date" .GitInfo.AuthorDate.Local "Format" .Site.Params.BookDateFormat) -}}
-    <a class="flex align-center" href="{{ .Site.Params.BookRepo }}/commit/{{ .GitInfo.Hash }}" title='{{ i18n "Last modified by" }} {{ .GitInfo.AuthorName }} | {{ $date }}' target="_blank" rel="noopener">
+    {{- $commitPath := default "commit" .Site.Params.BookCommitPath -}}
+    <a class="flex align-center" href="{{ .Site.Params.BookRepo }}/{{ $commitPath }}/{{ .GitInfo.Hash }}" title='{{ i18n "Last modified by" }} {{ .GitInfo.AuthorName }} | {{ $date }}' target="_blank" rel="noopener">
       <img src="{{ "svg/calendar.svg" | relURL }}" class="book-icon" alt="Calendar" />
       <span>{{ $date }}</span>
     </a>