Pull request template: mention doc_gram / refactor.
Pull request template: mention doc_gram / refactor.

Add a new bullet point mentionning doc_gram.
Refactor to make the template lighter.
Add pointers to all the relevant developer documentation.

Zimmi48/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq,coq/coq,coq/coq,Zimmi48/coq,coq/coq,Zimmi48/coq,coq/coq,SkySkimmer/coq,Zimmi48/coq,SkySkimmer/coq,ejgallego/coq,SkySkimmer/coq,ejgallego/coq