Doc improvements

[ukkopahis]

Collection of purely documentation improvements:

• docs/Updates
  • Add how to do Raspberry OS "apt update"
  • Change full update procedure to recreate from templates
  • Clarify wording
• docs: autoselect initial color-scheme. If user has browser with dark-mode preference, automatically select it.
• docs/Getting Started: Small fixes and improvments
  • Reword volume-mount section for better clarity
  • Fix scripted install section to link to PiBuilder (was linking to deprecated gist)
  • Convert "patch 2" steps to numbered list for better readability
• docs: add changelog
• docs: move network migration to its own page