[doc] Document all directives and roles of our Sphinx domain
[doc] Document all directives and roles of our Sphinx domain

Also get rid of a few unused or redundant constructs: the :ltac: role and the
'tac' directive (unused) and the :gallina: and :notation: roles (redundant).

gares/coq,coq/coq,ppedrot/coq,ejgallego/coq,maximedenes/coq,Zimmi48/coq,silene/coq,maximedenes/coq,ppedrot/coq,JasonGross/coq,Matafou/coq,maximedenes/coq,SkySkimmer/coq,Zimmi48/coq,gares/coq,JasonGross/coq,silene/coq,JasonGross/coq,gares/coq,ppedrot/coq,ppedrot/coq,JasonGross/coq,gares/coq,SkySkimmer/coq,ejgallego/coq,ppedrot/coq,SkySkimmer/coq,coq/coq,Zimmi48/coq,coq/coq,Matafou/coq,maximedenes/coq,ejgallego/coq,Matafou/coq,Zimmi48/coq,coq/coq,maximedenes/coq,silene/coq,gares/coq,SkySkimmer/coq,ejgallego/coq