Add and consolidate errors and warnings for async and await

[BillWagner]

Contributes to #49292.

One of the new diagnostics is for async 2. As part of adding that, consolidate and add any existing diagnostics related to async and await.

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Internal previews

📄 File	🔗 Preview link
.github/prompts/error-consolidation.md	Copilot prompts to consolidate error codes.
docs/csharp/language-reference/compiler-messages/async-await-errors.md	Resolve errors and warnings in async methods using the await operator
docs/csharp/language-reference/keywords/async.md	"async keyword"
docs/csharp/language-reference/toc.yml	docs/csharp/language-reference/toc
docs/csharp/misc/sorry-we-don-t-have-specifics-on-this-csharp-error.md	C# 14 errors begin here

[Copilot]

Pull Request Overview

This PR consolidates multiple async/await-related error and warning documentation files into a single comprehensive document. The goal is to organize 18 different compiler errors (CS1983-CS9330) related to async methods and await expressions into one centralized location with better categorization.

• Consolidates 10 individual error files (CS1983, CS1986, CS1994, CS1996, CS1997, CS4008, CS4009, CS4014, CS4032, CS4033) into a new unified document
• Organizes errors into three logical sections: await expression requirements, async method signature requirements, and async practices
• Updates TOC, redirections, error lists, and internal documentation to reflect the consolidation

Reviewed Changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
async-await-errors.md	New consolidated documentation covering 18 async/await errors organized into three categories
toc.yml	Adds new async errors section and removes individual error entries, consolidating them into one navigation item
sorry-we-don-t-have-specifics-on-this-csharp-error.md	Removes consolidated error codes from the "undocumented" list and adds blank line before C# 14 section
async.md	Updates link from individual CS4014 page to consolidated async-await-errors page
.openpublishing.redirection.csharp.json	Adds 10 redirections from old individual error pages to new consolidated page
error-consolidation.md	Updates prompt template to reference async/await errors instead of using statement errors
CS4009.md, cs4033.md, cs4032.md, cs4014.md, cs4008.md, cs1997.md, cs1996.md, cs1994.md, cs1986.md, cs1983.md	Deletes individual error files that are now consolidated

[meaghanlewis]

@BillWagner these changes LGTM with the exception of the feedback Copilot left in async-await-errors.md. The 3 latter comments seem like good feedback.