icons for header menu

[narrenfrei]

add icons to header menu items like explained in Hugo docs (https://gohugo.io/content-management/menus/#add-non-content-entries-to-a-menu)

[narrenfrei]

I've published a preview of my changes:
http://docsy-dev-4.narrenfrei.at

In my PR I've also added icons to the header menu icons of the userguide. I don't know, if this is wanted or not. You can delete them of course.

This PR is part of my suggestion for #457

[LisaFC]

Oh this is really nice! Made some minor updates to the docs section. Are there any dependencies here on other PRs or can we merge this now?

[narrenfrei]

Oh this is really nice! Made some minor updates to the docs section. Are there any dependencies here on other PRs or can we merge this now?

No, there are no dependencies. I pack the PRs in such a way that there should be no dependencies. Wherever dependencies arise, I wait with further PRs until required PRs have already been merged.

For your info, these PRs are planned from my side:

1. Restructuring of the sidebar menu: rebuilt sidebar menu structure #475 (MERGED) and corrected sequence of A and SPAN tag in restructured sidebar menu #483
2. Icons in header menu: icons for header menu #480 (this one)
3. Icons in sidebar menu: Icons for sidebar menu - WIP #485 (WIP, feedback needed)

When #483 and #485 are merged:

4. External links: Add external links to sidebar #449
5. Foldable Sidebar Menu: Toggle expand/collapse for left-side menus/TOC #342
6. JS: scrolling to active sidebar menu item: Left-side menu / TOC jumps around after you click a menu #348
7. some CSS tweaks
8. Some tweaks for Tagging as update to Issue #272 Add tagging support #324

[LisaFC]

Perfect! I'm happy to merge this one now.