Documentation: update the TypeSpec directory structure guidance in the specs repo

[konrad-jamrozik]

A follow-up to:

• Update the specification directory structure article azure-rest-api-specs#28464
• Fixups to dir structure docs azure-rest-api-specs#28606

As requested by @weshaggard:

• Update the specification directory structure article azure-rest-api-specs#28464 (comment)

The article:

• https://github.com/Azure/azure-rest-api-specs/blob/main/documentation/typespec-structure-guidelines.md

Needs to be updated to:

• reflect the updated dir structure article at https://aka.ms/azsdk/spec-dirs
• change the tense to reflect this is no longer a proposal but the current state of things
• change service family to service group. See this discussion

[josefree]

I wonder if you are going to retire this guidance https://github.com/Azure/azure-rest-api-specs/blob/main/documentation/typespec-structure-guidelines.md when https://aka.ms/azsdk/spec-dirs is refreshed?