asciidoc: add clickable anchor link to all headings

asciidoc: add clickable anchor link to all headings

Works the same as in our PBS documentation and is generally common for
documentations.

Very useful for linking specific sections of the documentation in other
places. Previously, this always had to be done by getting the correct
anchor from the HTML directly via e.g. browser devtools.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>
Tested-by: Max Carrara <m.carrara@proxmox.com>