Update default fixture to give better direction

closes #1561
- altered perspective of intro paragraph to reading from the blog
instead of from the content page
- added copy directing users to sign in to the admin area at /ghost/ and
edit the post before reading the 'Getting Started' section
- this should give the Markdown lessons better context and avoid users
getting confused as to what 'the left hand panel of Ghost' is
Merge pull request #1677 from nickpfisterer/default-fixture-update

Update default fixture to give clearer direction