Allow to add FA icons to page_links

[jpawlowski]

I would love to see that I could define icons for any button that is processed in page_links.html.

Today I need to make a copy of that file to add icons by myself which might not be as elegant as it could be.

Guess I am not the only person that would be interested to make the content more obvious to distinguish besides the text labels. :-)

[gcushen]

This is already supported by the links option. By not using the automated linking features, all icons can be set with links. Also refer to #765 and similar issues.

[jpawlowski]

I'd like you considering to reopen this request as links does not work for every use case.
For example, if you link to a project page using the project: YAML tree, the project page does not get any icon without modifying page_links.html. It would be the same situation for other content types (e.g. slides).

Also, do you consider url_* to be deprecated because links was introduced? If that was the case, I suggest that the template file for the relevant content types is updated here. Most people (like me) will tend to stick to the url_* defaults as the comment indicates that links: shall only be used for custom links where there is no pre-defined content type available from the given list below.