Add links to the API documentation and man-pages

[kit-ty-kate]

Related to ocaml/opam#6637

Previously these directories were only partially filled as only the files linked elsewhere in the website were copied over from the /usr/local/share/opam2web/content directory.

The opam2web menu API can work in two ways based on whether menu_item is External or Internal/Submenu.

• In External mode nothing is copied over, only files mentioned elsewhere in one of the parsed markdown files are
• In Internal mode one has to give a link to the main index file (content of a html file, not full html) which links the files to copy over
• The Submenu mode is similar to Internal the whole list of links will be part of the menu itself

Published in staging.opam.ocaml.org