Handles the user help pages.

• Accessed via index.php?action=help or index.php?action=help;page=x
• Uses the "help" stylesheet (passes appropriate tag via $context['html_headers'])
• Uses Help template and Manual language file
• "page" corresponds to a key in $context['all_pages']
• If "page" isn't valid or isn't specified, the main (index) help page is loaded
• Values of $context['all_pages'] indicate which "manual_" subtemplate to use for the specified page