PPTX writer: support notes field in metadata for title slide (#11396)

This adds support for a `notes` field in the YAML metadata block
that will be used as speaker notes for the title slide in PowerPoint
output.

Previously, there was no way to add speaker notes to the title slide
since it is generated from metadata rather than from content blocks.
The `::: notes` syntax only works for content slides.

Example usage:

    ---
    title: My Presentation
    notes: |
      Welcome everyone to this presentation.
      Remember to introduce yourself.
    ---

Closes #5844 (for PPTX output).

Co-authored-by: Chris Callison-Burch <[email protected]>

1 files changed, 17 insertions, 0 deletions

diff --git a/MANUAL.txt b/MANUAL.txt
index 28c6341d0..28d4aa160 100644
--- a/MANUAL.txt
+++ b/MANUAL.txt
@@ -6938,6 +6938,23 @@ in handouts and presenter view.
 Notes are not yet supported for other slide formats, but the notes
 will not appear on the slides themselves.
 
+### Speaker notes on the title slide (PowerPoint)
+
+For PowerPoint output, the title slide is generated from the document's
+YAML metadata block. To add speaker notes to this slide, use a `notes`
+field in the metadata:
+
+    ---
+    title: My Presentation
+    author: Jane Doe
+    notes: |
+      Welcome everyone to this presentation.
+
+      Remember to introduce yourself and mention the key topics.
+    ---
+
+The `notes` field can contain multiple paragraphs and Markdown formatting.
+
 ## Columns
 
 To put material in side by side columns, you can use a native