Add indicator and explanatory note for ms part results in browse/search

[wlpotter]

Splitting this off from #428, which is otherwise done.

For results which are ms parts, we want to include an explanatory note saying that they are a part of a composite manuscript, with a link to a documentation page explaining this further. Steps for this:

• Add to the JSON index a field marking which records are parts (and their parent shelf mark?)
• Determine how to display this info to the user
• Create the documentation page explaining composite mss, etc.
• Link to the page from search results for parts