Merge: doc/commands: introduce CmdEntityLink command

This PR introduce the CmdEntityLink command.

The link command is used to create a link from the generated documentation file to an external resource.

Command forms:
* `[[popcorn]]` generates a link to the `popcorn` package documentation resource.
* `[[popcorn | text: foo, title: bar]]` allows customization of link text and title.

The resource URL depends on the tool.

For example:
* `nitweb` (when the command is used in `docdown` or in a `README`) renders a link to the HTML documentation page on the server
* `nitdoc` (TODO) renders a link to the static documentation page
* `nitx` replaces the link commands by the comment commands (that contains the file location)

Pull-Request: #2641
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>