Improve welcome message

[Anders Logg (Chalmers)]

• Nicer formatting
• Include changeset and timestamp
• Updated instructions for finding demos
• Hint on sharing data (-v)?

[Jack Hale]

• I'm happy with formatting, are you?
• Timestamp of build is available in docker inspect. Changesets available easily using git in build/src.
• Done in stable.
• Done in documentation and fenicsproject script.

[Prof Garth Wells]

Is this done, and the issue ready to be closed?

[Anders Logg (Chalmers)]

Looks good now.