The previous built-in reference doc had only title and content
layouts. Add in a section-header slide and a two-content slide, so
users can more easily modify it to build their own templates.
Golden files needed to be regenerated. Checked on MS PowerPoint 2013.
* docx writer: support custom properties. Solves the writer part of #3024.
Also supports additional core properties: `subject`, `lang`, `category`,
`description`.
* odt writer: improve standard properties, including the following core properties:
`generator` (Pandoc/VERSION), `description`, `subject`, `keywords`,
`initial-creator` (from authors), `creation-date` (actual creation date).
Also fix date.
* pptx writer: support custom properties. Also supports additional core
properties: `subject`, `category`, `description`.
* Includes golden tests.
* MANUAL: document metadata support for docx, odt, pptx writers
These were never added when the tests were first created.
Output files checked in MS PowerPoint 2013 (Windows 10, VBox). No
corruption, and output as expected.
