Add config for HMRC manuals

To allow adding HMRC manuals to GOV.UK search.
Sorry - I relied on my unreliable memory.  The field I was thinking of is simply `manual`, which is expected to contain the slug of the manual.  This is different from hmrc_manual_section_id, so you can discard my comment.  It might be good to add a "manual" field though, containing the slug of the manual, to allow searches to be performed which are filtered to only a particular manual.