Add documentation for `??` `??=` `?.` and `?[]` operators

[adityapatwardhan]

Summary of the new document or enhancement

Related to PowerShell/PowerShell#10636

Details of requested document:

• Proposed title: Null coalescing operators
• Propose location in the TOC:
• Target audience: developers
• Purpose or scenario: $x ?? $y and $x ??= 10
  or
  Proposed changes/additions to existing article:
• List of related articles to link to:

[sdwheeler]

The null-coalescing operator ?? returns the value of its left-hand operand if it isn't $null. Otherwise, it evaluates the right-hand operand and returns that result. The ?? operator doesn't evaluate the right-hand operand if the left-hand operand evaluates to non-null.

The null-coalescing assignment operator ??= assigns the value of its right-hand operand to its left-hand operand only if the left-hand operand evaluates to null. The ??= operator doesn't evaluate its right-hand operand if the left-hand operand evaluates to non-null.

@adityapatwardhan See https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-coalescing-operator. Can you provide similar use cases?