[openssl/web] 08d5c9: Fix problems with newer pandoc versions

[Richard Levitte]

  Branch: refs/heads/master
  Home:   https://github.com/openssl/web
  Commit: 08d5c9da50b3b1c764b4619d0528010d85c9d95a
      https://github.com/openssl/web/commit/08d5c9da50b3b1c764b4619d0528010d85c9d95a
  Author: Richard Levitte <levitte at openssl.org>
  Date:   2022-08-26 (Fri, 26 Aug 2022)

  Changed paths:
    M bin/md-to-html5
    A inc/styles.html

  Log Message:
  -----------
  Fix problems with newer pandoc versions

Pandoc absolutely wants a styles.html alongside the HTML5 template, so
we give it one that's empty.

Also, --base-header-level is deprecated and should be replaced with
--shift-heading-level-by.  The pandoc documentation gives us this
formula:

    Use --shift-heading-level-by=X instead, where X = NUMBER - 1

Reviewed-by: Matt Caswell <matt at openssl.org>
(Merged from https://github.com/openssl/web/pull/359)