docs: generate a gitlab pages with hugo

Followup of the documentation inclusion in asciidoc.

Few points to note:

• I tried to keep the current content and do a single sed for the URL to make the docs working both on gitlab and on gitlab pages
• the theme is the default one from geekdoc (MIT)
• a preview is available in the artifacts browser of the pages CI check job in the MR pipeline
• the acutal deployment should only happen on the main branch
• as long as Settings -> General -> Visibility, project features, permissions -> pages is set to Only project Members, gitlab authentication is required to access the docs.
• the order of the menu should probably be thought a little bit more, but... meh (please play with the various weight in the asciidoc files headers

I chose hugo over some other generator, because it's written in go, which means I know at least 2 RH kernel engineers capable of reading the code 👀

Signed-off-by: Benjamin Tissoires benjamin.tissoires@redhat.com