Poor Language reference structure

[bernardnormier]

The structure of the Language reference is not good:

• virtually all the useful information is on the Grammar specification page
• the information on the remaining pages is mostly or entirely redundant

Furthermore, it's impossible to reference specific syntax on the grammar page:
https://docs.testing.zeroc.com/slice2/language-reference/grammar

because it uses gigantic code blocks such as:
https://docs.testing.zeroc.com/slice2/language-reference/grammar#lexical-grammar
https://docs.testing.zeroc.com/slice2/language-reference/grammar#syntactic-grammar

I think it would make much more sense to:

• "merge up" the grammar specification, convert each section on the grammar page into a separate page: Lexical Rules, Grammar notation, Symbol conventions, Core languages, Preprocessor directives etc
• delete the other pages while salvaging any useful information in them
• introduce additional titles and levels to allow referencing

For example, if you look at the C# spec, you can reference
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/types#82-reference-types
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/types#83-value-types

It's not one giant ANTLR block with everything in it.

[InsertCreativityHere]

The language reference has been temporarily deleted. See #304.