Add pptx slide-deck build target for screen-share recordings

Same markdown now also builds a PowerPoint deck (make slides /
make template.pptx) alongside the existing PDF, for recording
walkthrough videos. Styling comes from reference.pptx since pandoc's
pptx writer ignores style.css; its theme was hand-tuned to match the
PDF's dark palette and fonts.

Restructure the closing block into a two-column div so the author
photo survives in slide exports (pandoc drops images sharing a plain
div with paragraph text), with a CSS override so the PDF keeps its
original centered layout.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

template.md

@@ -36,9 +36,12 @@ who needed your stories already found them.
 
 ![](images/step-3-computer.jpg)
 
-::: {.closing}
+:::: {.closing .columns}
+::: {.column width="30%"}
 ![](images/author-photo.jpg){.author-photo}
+:::
 
+::: {.column width="70%"}
 By: Will Estes — ReGround
 
 **Are you wondering how to get your stories in front of readers who need them?**
@@ -49,3 +52,4 @@ your stories to reach the readers who need them.
 
 [Get a Free Strategy Session](https://reground.org/talk){.cta-button}
 :::
+::::