Update download instructions

Download instructions used to prompt users to log in to ghost.org and then click a download button. Since registration isn't required to download Ghost and the download button isn't even visible to Ghost Pro subscribers, download instructions now:

- Omit mention of logging in to ghost.org
- Link directly to the download page at https://ghost.org/download/
Merge pull request #2770 from nickcernis/gh-pages

Update download instructions