Allow "description" in head.html partial to be configurable

[adambkaplan]

The current <meta name="description"> tag in the head partial is hard coded to the Lotus Docs theme description. This tag appears in applications like Slack that show link previews.

Users of the theme should be able to customize this value.

[khemarato]

Not just the description meta but all the head metadata: tags, author, etc.

[colinwilson]

Agreed. Will add these options in the next update.