*: introduce <pkg> tag for nicer package links

[arthurzam]

When I was writing the part for destabaling arch, I noticed we don't have any nice link/render for package (for the text "use qkeyword from app-portage/portage-utils").

I've decided to add a new tag <pkg> which renders in similar way to how we do it in Gentoo Wiki, with link to packages.g.o, a small nice "package" icon after it.

Then I changed all the places where we refer to specific package (and not as just a generic example or source for example). When a reader will want to go to the package page.

@ulm I'm find with rejecting this PR if you think this is useless or not needed.

[ulm]

I'm not yet sure if I like this. The element makes sense, but OTOH DevBook XML is supposed to be simple. So for example we have only a c element instead of code, var, key, path etc.

In any case, if we should accept this, can you please split this into two separate commits, one for the plumbing, and another for the text changes?

[ulm]

This won't work (and make validate fails) because the element has no parent. It must be added to inline.class (see line 11).

I also wonder whether it should be more general, so we could have <pkg>app-editors/emacs</pkg> or alternatively <pkg name="app-editors/emacs">The One True Editor</pkg>. The uri element does something similar.