Revamp “Getting Started” page (content + graphics)

[sunnydean]

• Text to introduce EarthCODE at the start of the documentation needs to be rewritten (Anca) to focus more on EarthCODE's offering rather than being generic
• Remove tutorial listings; replace with a link to the Examples page.
• Add a clear "Explore the Open Science Catalog" step to the EarthCODE System Overview.
• Delete final paragraph and tutorial references from Step 2.

[u3ks]

changed with #126