(pcase): Add buttons to the macros' defs in the docstring of `pcase`

(pcase): Add buttons to the macros' defs in the docstring of `pcase`

* lisp/emacs-lisp/pcase.el (pcase--find-macro-def-regexp): New var.
(find-function-regexp-alist): Add entry for `pcase-macro`s.
(help-fns--signature): Move declaration to where we know it is valid.
(pcase--make-docstring): Add buttons to jump to the definition
of Pcase macros.