Add description for each sub-command and generate manpages in the website

It would be nice to have a better description of the sub-commands, and make them available in the website.

This MR tries to do so using groff's output from cmdliner, and using an script based on pandoc for generating .md manual pages.

gitlab-pages/docs/manpages/changelog.md

+### NAME
+
+ligo-changelog - Dump the LIGO changelog to stdout.
+
+### SYNOPSIS
+
+**ligo changelog** \[*OPTION*\]\...
+
+### DESCRIPTION
+
+This sub-command dumps the changelog to the stdout.
+
+### OPTIONS
+
+**\--format**=*DISPLAY_FORMAT*, **\--display-format**=*DISPLAY_FORMAT* (absent=human-readable)
+
+:   *DISPLAY_FORMAT* is the format that will be used by the CLI.
+    Available formats are \`dev\`, \`json\`, and \`human-readable\`
+    (default). When human-readable lacks details (we are still tweaking
+    it), please contact us and use another format in the meanwhile.
+
+**\--help**\[=*FMT*\] (default=auto)
+
+:   Show this help in format *FMT*. The value *FMT* must be one of
+    \`auto\`, \`pager\`, \`groff\` or \`plain\`. With \`auto\`, the
+    format is \`pager\` or \`plain\` whenever the **TERM** env var is
+    \`dumb\` or undefined.
+
+**\--version**
+
+:   Show version information.