Update coding guidelines

[BillWagner]

There are a number of issues related to our coding standards used in .NET docs.

Many of the issues arise because readers conflate our style with Microsoft recommended style. The first commits made that distinction much more clear.

This may be easier to review commit-by-commit:

1. Rearrange the content. Some naming conventions were used in both files, clearly separate them. Rearrange the order.
2. Rework the content. Make it clear that these are our team guidelines. Point to the source for the runtime and roslyn repos. Add appropriate disclaimers that readers should feel free to modify the guidelines to suit their team's needs. Finally, add a section to explain the tools where readers can use editorconfig or analyzers to enforce the rules they want.
3. Fix issues related to var:
   • Fixes Review guidance on implicit typing for local variables #26787: clarify "obvious"
   • Fixes Explanation of use of var and variable names is generally a bad practice. #32633: add explanation, update variable names.
   • Fixes Clarify usage of implicit typing for nested generic types / LINQ. #34940: Explain that var is preferred in LINQ queries, despite other rules.
4. Fix issues related to naming conventions:
   • Fixes In visual studio private variables have no _ #30626: Clarify (again) that these are our guidelines, not yours. Point out that it's not the VS default, but a configuration option.
   • Fixes Naming style for static members contradicts with the page "Names of Type Members" #30642: Again, our style.
   • Fixes Clarify naming style for const fields #30799: Change constant style from ALL_CAPS to PascalCase to match runtime repo.
   • Fixes Inconsistent naming conventions for delegate instances #33959: Update variable names so delegate types are PascalCased and instances of a delegate are camelCase. Add clarifying text for the same.
5. Fixes #31951 : Rewrite the exception example so that it's still obvious what can fail, but couldn't be easily anticipated before making the computation.
6. Fixes Consider moving generic type parameter naming guidelines to general C# coding conventions #30897: Move the Generic type parameter naming conventions to the general naming conventions.
7. Fix build issues.

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Internal previews

📄 File	🔗 Preview link
docs/csharp/fundamentals/coding-style/coding-conventions.md	Common C# Coding Conventions
docs/csharp/fundamentals/coding-style/identifier-names.md	"C# identifier names"
docs/csharp/programming-guide/generics/generic-type-parameters.md	Generic type parameters (C# Programming Guide)

[IEvangelist]

Just a few nits, otherwise this looks great!