I'm new using KDE 5 and I would like to add custom actions to Dolphin but I don't find any documentation about this.

You don't have to look far...

raphaelra wrote:You don't have to look far...

That isn't a tutorial nor documentation about create .desktop service files.

It contains enough info about how to add an menu entry (which was your question). There is also an outdated article on KDE-Techbase, but it may be helpfull as well.

For creating desktop entries/files, you may consult the specification or choose one description/article found on google.

It would be a good idea that someone updates the documentation to KF5 and Dolphin.
Another problem is that in that documentation it only talks about one of the X-KDE-... entries (not included in the standard).
I have found this but there is outdated too.

ok, thanks. If that's all there is, I'll try to deal with it.

"X-KDE-RequiredNumberOfUrls" serves to specify an exact number of selected files for the submenu to be displayed.
But what if I want to indicate a minimum but not a maximum?

I have created this Bug #418492.

Here some info about using X-KDE-Priority and X-KDE-Submenu.

UPDATE

I did some digging in the Dolphin source code and found out that there is an API that collects the service actions for a file/directory and can add them to the menu. It can be also used in other apps supporting browsing in files/directories (Kate, Gwenview, ...).

If you are familiar with C++, you can take a look at methods addServiceActionsTo() and addPluginActionsTo(). You will find there, which X-KDE-* entries are utilized and how ... I didn't know for example that the service actions could also be defined in the .directory file

To comment on "out-of-date" documentation, the specifications at play haven't really changed much if at all since KDE 4 (or earlier), so even if some of it still refers to KDE 4 (or the need to update to KDE/plasma 5), the information in them should still be relevant.

(though i will note that one difference you may run into is wrt file locations for some of this stuff which moved from ~/.kde(4) to ~/.config or ~/.local/share, employing various XDG_*_DIR(s) environment variables)