Remove dead link from install documentation

Summary: XHProf install documentation went missing a month or two ago (see T725)
and doesn't work in the widely deployed versions of PEAR/PECL. Provide
build-from-source instructions inline.

Test Plan: Generated, read documentation.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T725

Differential Revision: https://secure.phabricator.com/D1345