Change example assets in the documentation

The existing examples of assets are not the best ones. These are better representation of what our users might expect when we talk about an asset list.
LMW-97 Polish home page. Add direct links to inner content.
LMW-97 Nicer image and other minor changes
LMW-97 Empty lines
LMW-97 New separated guides for screenlet and theme creation
LMW-97 Changed images where “widget” word was mentioned
Wordsmithing.
Wordsmithing for additional files.
Merge pull request #51 from liferay/LMW-97-polisher-doc

LMW-97 Polisher doc