2.4: Update docs for release

[degville]

• 2.4: top-level navigation changes #856 top-level navigation changes 2.4 5
• 2.4: KVM over-commit ratios for CPU and memory #855 KVM over-commit ratios for CPU and memory
• 2.4: maas-dns package deprecated #854 maas-dns package deprecated
• 2.4: ntpd deprecated, NTP services provided by Chrony #853 ntpd deprecated, NTP services provided by Chrony
• 2.4: web UI screenshot refresh #852 web UI screenshot refresh 2.4 13
• 2.4: top-level What's New in 2.4 doc #851 top-level What's New in 2.4 doc
• 2.4: Add details on Machine Locking #850 Add details on Machine Locking
• 2.4: Remove references to deprecated Apache #849 Remove references to deprecated Apache
• 2.4: Add upgrade PostgreSQL doc to toc #848 Add upgrade PostgreSQL doc to toc
• 2.4: maas init --mode all no longer an option #847 maas init --mode all no longer an option
• 2.4: New options when commissioning a machine #846 New options when commissioning a machine
• 2.4: settings in web UI are now organised into multiple tabs #858 settings in web UI are now organised into multiple tabs
• 2.4: pods can have a default storage pool defined #859 pods can have a default storage pool defined