Slightly better intro.

The "micro web application framework" mention sounds slightly negative - as
though Dancer is too small for use on serious apps, which is not the case.
Extend introduction somewhat.
typo fix, and explain bootstrapping.

Newbies might not understand what we mean by "bootstrapping", but "starting a
new app" is much more beginner-friendly)
Overhaul HTTP methods section.

In particular, add PATCH, and link to Dancer2::Manual::DSL entries for the
get/post/put/patch/delete keywords.