Reorganize the C# Guide around a new C# hub page

[BillWagner]

Fixes #39820
Fixes #40154

1. Create sub-TOC for specifications, along with a landing page for language specs.
2. Create sub-TOC for language reference, along with a landing page for the language reference. This required also updating or removing links to the previous index page. The links to the index was removed from pages in the language reference. In other areas, the link was updated to point at the new YML file.
3. Incorporate compiler errors messages into the language reference TOC.
4. Create new C# hub page.

Primary files changed:

• New C# Hub page
• Language reference landing page
• C# Language Spec landing page

The two new landing pages move that content out of the main conceptual TOC, which makes it more stand-alone. readers are less likely to fall into it by accident.

Most of the remaining files changed by removing a link to the overview page of the language reference. That seems redundant for language reference pages, as that page is the top page in the TOC.

---

Internal previews

Toggle expand/collapse

📄 File	🔗 Preview link
docs/csharp/fundamentals/exceptions/exception-handling.md	Exception Handling (C# Programming Guide)
docs/csharp/language-reference/builtin-types/bool.md	"bool type - C# reference"
docs/csharp/language-reference/builtin-types/built-in-types.md	Built-in types (C# reference)
docs/csharp/language-reference/builtin-types/char.md	char (C# reference)
docs/csharp/language-reference/builtin-types/default-values.md	docs/csharp/language-reference/builtin-types/default-values
docs/csharp/language-reference/builtin-types/enum.md	Enumeration types (C# reference)
docs/csharp/language-reference/builtin-types/floating-point-numeric-types.md	Floating-point numeric types (C# reference)
docs/csharp/language-reference/builtin-types/integral-numeric-types.md	Integral numeric types (C# reference)
docs/csharp/language-reference/builtin-types/nullable-reference-types.md	"Nullable reference types - C# reference"
docs/csharp/language-reference/builtin-types/nullable-value-types.md	Nullable value types (C# reference)
docs/csharp/language-reference/builtin-types/numeric-conversions.md	Built-in numeric conversions (C# reference)
docs/csharp/language-reference/builtin-types/record.md	docs/csharp/language-reference/builtin-types/record
docs/csharp/language-reference/builtin-types/ref-struct.md	ref structure types (C# reference)
docs/csharp/language-reference/builtin-types/reference-types.md	Built-in reference types (C# reference)
docs/csharp/language-reference/builtin-types/struct.md	"Structure types - C# reference"
docs/csharp/language-reference/builtin-types/unmanaged-types.md	Unmanaged types (C# reference)
docs/csharp/language-reference/builtin-types/value-tuples.md	Tuple types (C# reference)
docs/csharp/language-reference/builtin-types/value-types.md	Value types (C# reference)
docs/csharp/language-reference/builtin-types/void.md	"void - C# reference"
docs/csharp/language-reference/keywords/abstract.md	abstract (C# Reference)
docs/csharp/language-reference/keywords/access-modifiers.md	docs/csharp/language-reference/keywords/access-modifiers
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/ascending.md	"ascending - C# Reference"
docs/csharp/language-reference/keywords/index.md	C# Keywords
docs/csharp/language-reference/keywords/out.md	out (C# Reference)
docs/csharp/language-reference/tokens/index.md	C# Special Characters
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	Sorry, we don't have specifics on this C# error
docs/csharp/programming-guide/classes-and-structs/implicitly-typed-local-variables.md	docs/csharp/programming-guide/classes-and-structs/implicitly-typed-local-variables
docs/csharp/specification/overview.md	C# standard specification

Note
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.

[katiesavage]

Suggested learning resources for the CTA are:

• C# Certification
• C# Learning paths
• C# Beginner Video series on Learn or on YouTube

[katiesavage]

I also wonder if we should add the 'concepts' from the original C# index page into the "language concepts" section:

[BillWagner]

The MS Docs build verifier error is false positive: It's because index.md changed to index.yml in the C# root folder. Adding a redirection for that fails the OPS build.

[BillWagner]

@jamesmontemagno @claudiaregio @katiesavage @KathleenDollard @raprice

This is ready to merge. I tagged all of you for review if you want to take another look.

[katiesavage]

One idea - for the link to VS Code, should we send users to the C# get started page? This page includes instructions about installing the C# Dev Kit extension

[gewarren]

Looks great!

[BillWagner]

Thanks a lot @gewarren That was awesome!