Update quickstart for generator-langium

[luan-xiaokun]

Currently the template of generator-langium includes a validator service demo and a cli generator demo, both of which are related to the Hello-World grammar. Users often forget to delete/comment them after changing the .langium grammar file and get compiling errors (at least for me).

Updating the quickstart guide in the template may help improving user experience.

[msujew]

I agree, we should keep the validation there for users who are new to Langium, but put it in a comment block so it doesn't prevent compilation after changing the grammar.

[spoenemann]

The CLI will be made optional in #954