Update "About MetaCPAN" text to mention the view and mirror of CPAN

Sparked by a discussion on IRC (#debian-perl on OFTC) about if MetaCPAN is a canonical view of CPAN or not, it has been noticed that "About MetaCPAN" just mentions that MetaCPAN is a search engine but not that it's actually more than a search engine. I missed the "view" part which displays CPAN distribution content and @shadowcat-mst suggested to also mention its CPAN mirror.

The attached patches add mentions of these previously unmentioned parts of MetaCPAN and split the first paragraph into two paragraphs.