Add Windows 10 21H2 documentation.

Added:

• Used 20H2 as base and revalidated for 21H2.
• Added RTC/UTC clock for Windows dual boot.
• Added git commands for rebasing historical commits.

Changed:

• Dropped regedits when possible.
• Updated settings to be reflective of out-of-box usage; versus strict
  lockdown (e.g. camera, microphone are enabled but enough info is there
  for a user to disable if wanted).
• Moved 10 base to Windows base install.
• Separated base advanced settings into separate files.
• Updated SNMP requirements for Brother printer.

Removed:

• Removed 1903 documentation.