doc: Update doc
build: fix `./configure` when nettle is not present

`PKG_CHECK_MODULES` will fail unless we give it an action to execute on
failure. When nettle is not available, we set `have_nettle` to `no`.
build: allow one to build out-of-tree documentation

It is somewhat convenient to be able to build Wslay in a separated
directory:

```sh
mkdir build
cd build
../configure
make
```

Sphinx was looking for its source files in the build directory instead
of the source directory. We fix this.
Merge branch 'fix/no-nettle' of https://github.com/vincentbernat/wslay into vincentbernat-fix/no-nettle
Merge branch 'fix/out-of-tree-sphinx' of https://github.com/vincentbernat/wslay into vincentbernat-fix/out-of-tree-sphinx