remove programming guide index

[BillWagner]

Contributes to #39819

This file doesn't have any real content. It's also stale.

The links in the index.md file are all to items in this section anyway.

As for links to this page, remove them from the C# guide content. The node is easily findable in the TOC. In other areas, add a link to the first article in the section of the TOC.

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📄 File	🔗 Preview link
docs/csharp/how-to/concatenate-multiple-strings.md	How to concatenate multiple strings (C# Guide)
docs/csharp/how-to/parse-strings-using-split.md	"Divide strings using String.Split (C# Guide)"
docs/csharp/how-to/search-strings.md	How to search strings
docs/csharp/language-reference/index.md	docs/csharp/language-reference/index
docs/csharp/language-reference/keywords/abstract.md	abstract (C# Reference)
docs/csharp/language-reference/keywords/access-modifiers.md	"Access Modifiers - C# Reference"
docs/csharp/language-reference/keywords/accessibility-domain.md	Accessibility Domain (C# Reference)
docs/csharp/language-reference/keywords/accessibility-levels.md	Accessibility Levels (C# Reference)
docs/csharp/language-reference/keywords/base.md	base (C# Reference)
docs/csharp/language-reference/keywords/class.md	docs/csharp/language-reference/keywords/class
docs/csharp/language-reference/keywords/event.md	event (C# reference)
docs/csharp/language-reference/keywords/extern-alias.md	extern alias (C# Reference)
docs/csharp/language-reference/keywords/extern.md	"extern modifier - C# Reference"
docs/csharp/language-reference/keywords/file.md	file (C# Reference)
docs/csharp/language-reference/keywords/get.md	docs/csharp/language-reference/keywords/get
docs/csharp/language-reference/keywords/init.md	init (C# Reference)
docs/csharp/language-reference/keywords/interface.md	:::no-loc text="interface"::: (C# Reference)
docs/csharp/language-reference/keywords/internal.md	"internal - C# Reference"
docs/csharp/language-reference/keywords/new-constraint.md	new constraint (C# Reference)
docs/csharp/language-reference/keywords/new-modifier.md	docs/csharp/language-reference/keywords/new-modifier
docs/csharp/language-reference/keywords/partial-type.md	partial type (C# Reference)
docs/csharp/language-reference/keywords/private-protected.md	"private protected - C# Reference"
docs/csharp/programming-guide/statements-expressions-operators/equality-comparisons.md	Equality comparisons (C# Programming Guide)
docs/framework/performance/performance-tips.md	.NET Performance Tips
docs/standard/assembly/create-signed-friend.md	docs/standard/assembly/create-signed-friend
docs/standard/assembly/create-unsigned-friend.md	How to: Create unsigned friend assemblies
docs/standard/assembly/embed-types-visual-studio.md	Walkthrough: Embed types from managed assemblies in Visual Studio
docs/standard/assembly/friend.md	"Friend assemblies"
docs/standard/assembly/identify.md	How to: Determine if a file is an assembly
docs/standard/assembly/load-unload.md	docs/standard/assembly/load-unload

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This table shows preview links for the 30 files with the most changes. For preview links for other files in this PR, select OpenPublishing.Build Details within checks.