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/ligo.md

@@ -130,11 +130,11 @@ https://gitlab.com/ligolang/ligo/issues/new
 
 **transpile-contract**
 
-:   Subcommand: Transpile a contract to another syntax.
+:   Subcommand: Transpile a contract to another syntax (BETA).
 
 **transpile-expression**
 
-:   Subcommand: Transpile an expression to another syntax.
+:   Subcommand: Transpile an expression to another syntax (BETA).
 
 ### OPTIONS