Use Spine site-commons

[JuliaEvseeva]

This PR brings the common layouts, fonts, styles, and configurations from the site-commons.

Also, the repository imports other documentation modules and builds them together.

New imports can be added in the docs/hugo.toml file like this:

[params]
  # Specifies the order of the sidenav modules.
  moduleOrder = ["framework", "validation", "compiler"]

[module]
  [[module.imports]]
    path = 'github.com/SpineEventEngine/validation/docs'
    disable = false
  [[module.imports]]
    path = 'github.com/SpineEventEngine/framework/docs'
    disable = false
  [[module.imports]]
    path = 'github.com/SpineEventEngine/compiler/docs'
    disable = false

The README.md will be updated in another PR.