add `describe()` and `it()` to package index

[dmuenz]

The functions describe() and it() are not listed on the package index -- neither on the website (https://testthat.r-lib.org/reference/) nor within R (help(package = "testthat")).

There is a help page (https://testthat.r-lib.org/reference/describe.html and ?testthat::describe), but it's hard to discover.

[dmuenz]

@keywords internal is used in the Roxygen2 comment for describe(), so that explains why the functions are not in the package index. But is this intentional? It seems like describe() and it() are meant to be user-facing.