Mate markdown docu

Hi, 
Continuing the effort to make mate documentation based on markdown and html, I converted the documentation for pluma to markdown+html. 

I also made an effort to enable a build of the markdown documentation and separated the CSS template and appendix from the main documentation. 

Now, it would be nice if the build process would run the following command which is documented in:
 help/C/readme.txt

We (more specifically: package maintainers) should then add `pandoc` as a build dependency.