Add range operator as substring alternative in divide-up-strings article

[Copilot]

The "Extract substrings from a string" article only covered String.Substring—it didn't mention the C# range/index-from-end operators, which are often the cleaner choice for position-based extraction.

Changes

• New "Ranges and indices" section — explains .. and ^ operators as a direct String.Substring alternative with two examples:
  • Extracting from start, end, and middle of a string
  • Trimming a known-length suffix (e.g., file extension): filePath[..^3]
• Intro bullet list — added entry pointing to the new section
• See also — added link to the C# "Indices and ranges" tutorial
• New snippet file range.cs — two RangeExamples methods backing the article examples

string filePath = "C:\\Users\\user1\\bin\\fileA.cs";
string trimmedPath = filePath[..^3];  // C:\Users\user1\bin\fileA

Includes a note that VB doesn't support this syntax and should continue using String.Substring.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Mention range operator for substrings</issue_title>
<issue_description>Update the article to suggest using the range operator instead of String.Substring. For example, this trims off the last 3 characters:

string trimmedPath = path[0..^3];

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Document Details

⚠ Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.

• ID: 22f46a14-5be8-358e-1c74-7c406c07d22d
• Version Independent ID: 0ae3185f-728f-092d-eae6-9e61e1dbf499
• Content: Separate strings into substrings
• Content Source: docs/standard/base-types/divide-up-strings.md
• Product: dotnet-fundamentals
• GitHub Login: @adegeo
• Microsoft Alias: adegeo</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Mention range operator for substrings #21774

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