Refactor wiki homepage template lookup.

Because the parent BP Docs plugin implemented theme compat mode, we also
want to make the change to use a template part.

However, we also want to be considerate to older users of the plugin that
had access to the specialized wiki sidebar widget.  Implementing theme
compat would remove the wiki sidebar widget.

Thus, to maintain backwards compatibility for older users, the plugin now
checks for a 'docs/index-wiki.php' template in the theme.

If the current theme has this template, the plugin will use this.

If the theme does not have this template, theme compat will be used.  A
new template part is used for theme compat - 'docs/home-wiki.php'.

Fixes #8.
Remove old bundled template as this is no longer used.

People new to BP Docs Wiki will now use BP 1.7's theme compat with the new
template part - 'docs/home-wiki.php'.

People that want to utilize the older full template format will have to
move 'wiki-home.php' to 'docs/index-wiki.php' in their theme's directory.
Streamline the CSS.