cmake: add support for building HTML and PDF docs

Note that for some reason there is this warning (that also exists with
autotools, added since curl-7_15_1-94-ga718cb05f):

    docs/libcurl/curl_multi_socket_all.3:1: can't open `man3/curl_multi_socket.3': No such file or directory

Additionally, adjust the roffit --mandir option to support creating
links when doing out-of-tree builds.

Ref: https://github.com/curl/curl/pull/1288