(#1074) Add docs for Assert-ValidChecksum

[vexx32]

Description Of Changes

• Add documentation for Assert-ValidChecksum
• Remove old documentation for Get-ChecksumValid
• Update redirects and old xrefs

CLI PR: chocolatey/choco#3525

Motivation and Context

Adding documentation for rewritten cmdlet

Testing

• I have previewed these changes using the Docker Container or another method before submitting this pull request.

Change Types Made

• Minor documentation fix (typos etc.).
• Major documentation change (refactoring, reformatting or adding documentation to existing page).
• New documentation page added.
• The change I have made should have a video added, and I have raised an issue for this.
  • Issue #

Change Checklist

• Requires a change to menu structure (top or left-hand side)/
• Menu structure has been updated

Related Issue

Part of #1074