[doc] Revise getting-started article
[doc] Revise getting-started article

- Remove the "Welcome to ..." phrase, it just adds noise to the document.
  The "Intoduction" article already explains what tabris is.
- Use emoji for site notes.
- Remove links to app stores, refer to the article on the developer app
  instead to eliminate redundancy.

Change-Id: I973d1fda54593a1d1bd34bf61d179946d2215967

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js