Adds headings for react and whenever
Adds headings for react and whenever

While keeping the index. It's level 3 indexing, although it shows up
at the same level in the headings. Contributes to improve a bit the
tutorial, thus refs #1906, and at the same time fixes #1552

perl6/doc,perl6/doc,jonathanstowe/doc,MasterDuke17/doc,dmaestro/doc,perl6/doc,jonathanstowe/doc,tbrowder/doc,MasterDuke17/doc,MasterDuke17/doc,stmuk/doc,dmaestro/doc,tbrowder/doc,tbrowder/doc,dmaestro/doc,MasterDuke17/doc,stmuk/doc,stmuk/doc,dmaestro/doc,dmaestro/doc,perl6/doc,stmuk/doc,jonathanstowe/doc,jonathanstowe/doc,jonathanstowe/doc,perl6/doc,tbrowder/doc,stmuk/doc