Resolve "2.4.0 ARM updates"

Closes #2969 (closed).

• 7e1ddb45 fix space in the release version

  This caused code blocks in the ARM to have spaces in filenames that in reality aren't there.

• f0349d61 revised the quickstart page

  • added link to the cloudsmith tarball page
  • added |cloudsmith_repo| substitution
  • added sphinx-tabs extension
  • changed yum to dnf because with the deprecation of CentOS 7, all remaining supported rpm distributions have dnf by default.
  • changed /path/to/your/kea6/config/file.json to the configuration file installed by default /etc/kea/kea-dhcp6.conf for easier startup
  • regenerated doc/sphinx/src/requirements.txt mainly to include sphinx-tabs
  • added sphinx-tabs to hammer.py

• 0f3e5abd update supported-systems list in hammer.py

• 2c739cb9 add docker quick start guide

• f909665b add two missing branch labels to an UML

  • slightly unrelated, but still on the topic of documentation