Removes redundant instructions from the platform-specific plages and the main "get the code" page on the sites page. The platform pages should have only non-redundant platform-specific information.

When I did this the Linux page was basically empty with a link the the prerequistes page, so I consolidated these two pages, leaving a placeholder in the old prerequisites page for now to avoid breaking links.

There was a lot of out-of-date information removed.

Review-Url: https://codereview.chromium.org/2014753002
Cr-Commit-Position: refs/heads/master@{#396071}