Update for focal

Current tasks

• Get xenial docs building again (on ubports/xenial branch)
  • ubports/development/core/lomiri-system-settings-online-accounts!23 (merged) to build docs again for Ubuntu.OnlineAccounts.Client
• Start building focal docs (on main branch) - pending local branch push and MR creation
  • UBports packaging needs -doc-html subpackage for qtfeedback.index(.gz), dropped previously due to ubports/development/core/packaging/qtfeedback@11e5ab2b
  • ubuntu-html5-theme isn't yet migrated to GitLab, simply dropped for now due to https://t.me/ubports_devel/51964
  • unity-webapps-qml isn't yet migrated to GitLab, see above
  • Rebase/cherry-pick old focal-bringup branch on top of latest ubports/xenial and continue where it left off, including trying to build as much docs as possible for previously commented ones since most should be fixed now
• Fix CI so ubports/development/core/focal-projectmanagement-missing-packages#62 can be closed, orchestrator.sh or CI config needs fixups - applicable to both xenial (and focal?)
  • We should setup GitLab CI instead as per @UniversalSuperBox's response to my query about the broken Jenkins builds: https://t.me/ubports_devel/64932
• Figure out why cpp docs cannot be made anymore (previously used for {libaccounts,u1db}-qt) - applicable to both xenial and focal
  • Generation stopped for xenial with !9 (42724479)
• What about deployment to live @ https://api-docs.ubports.com/? Do we keep xenial docs? What about migration of https://github.com/ubports/api-docs/tree/gh-pages? GitHub pages was used for old docs seemingly