Previously we used Setext (underlined) headings by default.
The default is now ATX (`##` style).
* Add the `--markdown-headings=atx|setext` option.
* Deprecate `--atx-headers`.
* Add constructor 'ATXHeadingInLHS` constructor to `LogMessage` [API change].
* Support `markdown-headings` in defaults files.
* Document new options in MANUAL.
Closes#6662.
Screen readers read an image's `alt` attribute and the figure caption,
both of which come from the same source in pandoc. The figure caption is
hidden from screen readers with the `aria-hidden` attribute. This
improves accessibility.
For HTML4, where `aria-hidden` is not allowed, pandoc still uses an
empty `alt` attribute to avoid duplicate contents.
Closes: #6491