Docs updates (error on index, modules, cleanup)

- In controller's read_page(), if $type is not 'application' or
'bonfire' and it wasn't found in the module_list, revert to $defaultType
('application'). This may need to be updated again (set to 'bonfire' or
another check against the $type added). This addresses an error where
$content was not set displayed when going to
http://servername/docs/index (rather than http://servername/docs or
http://servername/docs/).
- Note that the links on "Welcome to Bonfire" don't work when using
http://servername/docs (but do on /docs/ or /docs/index, because the
paths are relative)
- Moved strings displayed in the docs views to language file
- Updated sidebar to:
- display app-specific docs
- support multiple docs per module (the view was throwing out additional
docs if $module_docs contained multiple docs for a given module)
- Allow config setting of theme (falls back to the 'docs' theme)
- Cleaned up some documentation and moved some strings to variables in
the controller