docs: script to convert HTML manual pages to markdown

Script to convert recursively all .html files to .md (GitHub flavoured Markdown).

(see related OSGeo#3849)

utils/grass_html2md.sh

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+#!/bin/sh
+set -eu
+
+###############################################################################
+# Convert recursively all .html files to .md (GitHub flavoured Markdown)
+#
+# Dependencies:
+#    pandoc
+#    wget
+#
+# Author(s):
+#    Martin Landa, Markus Neteler
+#
+# Usage:
+#    If you have "pandoc" in PATH, execute for HTML file conversion in
+#    current directory and subdirectories:
+#      ./utils/grass_html2md.sh
+#
+# COPYRIGHT: (C) 2024 by the GRASS Development Team
+#
+#            This program is free software under the GNU General Public
+#            License (>=v2). Read the file COPYING that comes with GRASS
+#            for details.
+#
+###############################################################################
+
+# define $TMP if not present
+if test -z "${TMP}" ; then
+  TMP="/tmp"
+fi
+
+# TODO: path to LUA file setting to be improved (./utils/pandoc_codeblock.lua)
+#wget https://raw.githubusercontent.com/OSGeo/grass/refs/heads/main/utils/pandoc_codeblock.lua -O "${TMP}/pandoc_codeblock.lua"
+TMP="utils"
+
+# run recursively: HTML to MD
+for f in `find . -name *.html`; do
+    echo "${f}"
+    cat "${f}" | sed 's#<div class="code"><pre>#<pre><code>#g' | sed 's#</pre></div>#</code></pre>#g' | pandoc \
+        --from=html --to=markdown -t gfm --lua-filter "${TMP}/pandoc_codeblock.lua" | \
+        sed 's+  $++g' | sed 's+\.html)+\.md)+g' > "${f%%.html}.md"
+done

utils/pandoc_codeblock.lua

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+-- Pandoc Lua filter to handle code blocks
+-- Test cases
+-- raster/r.sun/r.sun.html
+
+-- Function to convert code blocks to markdown
+function CodeBlock (cb)
+    return pandoc.RawBlock('markdown', '```bash\n' .. cb.text .. '\n```\n')
+end